Every Monday, Tuesday, and Friday at 8:39 a.m., Tony Mao files into his sophomore-level University of Connecticut engineering course in applied mechanics. Mao is not a university student. He’s a high school senior at the private Christian Heritage School in Trumbull, Conn., who spends his free time playing on the coed varsity tennis team and writing college application essays.

Part of a skyrocketing number of students attending high school and college simultaneously, Mao (who got a perfect score on the SAT in math) plans to use the courses so he can skip ahead on his route to a career in architecture.

But it’s not just precocious high achievers like Mao who are benefiting from this phenomenon that is most often referred to as “dual enrollment” but is also known as “dual credit” or “concurrent enrollment.” Increasingly, educators and other advocates are touting dual enrollment as an effective way of helping underserved students—including first-generation college-goers, low-income racial minorities, and even dropouts—to obtain a college degree.

And while questions remain about access and quality, evidence is accumulating that students who take courses for college credit while in high school are more likely to go on to college, stay there, and ultimately graduate than classmates of similar backgrounds and abilities who follow the traditional high school path.

People assume that taking college courses while in high school “is only for top students in really good suburban high schools. But it’s not,” says Ben Byers, associate vice president for operations at Gateway to College, a nonprofit organization that places dropouts and those on the verge of dropping out in special programs on 43 college campuses across the country, where they take college and college prep courses.

The Growth of Dual Enrollment

The oldest statewide dual enrollment program dates to 1985, when Minnesota agreed to pay for high school students to take college courses. But the movement has picked up speed as more and more jobs require a college credential and students and families facing skyrocketing college tuitions look for options for reducing the time it takes to earn an associate or bachelor’s degree.

More than 2 million students in 15,000 high schools are now enrolled in college-level courses offered at, or in collaboration with, community colleges and other institutions—up 75 percent in the last decade, according to figures released in 2014 by the National Center for Education Statistics. Some high schoolers take these courses on college campuses or online, but three-quarters of the courses are taught by high school teachers in local schools. The courses, which range from sociology to advanced engineering, go beyond Advanced Placement (AP) and International Baccalaureate programs, for which some colleges have historically awarded credit toward degrees. In another incarnation known as “early college,” a network of 280 high schools uses a curriculum that melds high school and college work to offer low-income students, first-generation college-goers, English-language learners, and students of color the opportunity to earn up to two years of college credit while also earning their high school diploma.

What the Numbers Show

Studies conducted nationally and in states over the last few years paint a persuasive picture of the benefits of dual enrollment. An analysis in 2013 by the National Student Clearinghouse, which tracks enrollment and graduation rates in every state, found that two-thirds of all students who took dual enrollment courses while in high school earned a college or university degree within six years, compared with only half of traditional high schoolers.

In Texas, one of the few places where available data let researchers track students from high school through public colleges and universities, high schoolers who took college courses were more than twice as likely to go on to two- or four-year colleges, twice as likely to return for a second year, and almost twice as likely to earn degrees as those who didn’t, according to Jobs for the Future, a nonprofit research organization that advocates for multiple pathways to college. In Colorado, a study by that state’s Department of Higher Education found that dual enrollment students were less likely to need remedial classes when they got to college. And in Oregon, dual enrollment students are more likely to go on to college, stay there, and get higher grades, according to a 2010 study by the Oregon University System.

Notably, of the students attending early college high schools, 59 percent went on to community colleges and 54 percent to four-year universities, compared with 38 percent and 47 percent of their peers who applied but did not win lotteries to attend the early college schools, according to the 2013 results of a study by the independent, nonprofit American Institute for Research (AIR). Another study of early college high schools by Jobs for the Future determined that their graduation rates were also higher, with 84 percent of the students finishing high school compared with an average of 76 percent in their surrounding districts.

And Gateway to College reports that the dropouts and at-risk students who take dual enrollment courses through its programs earn an average of 33 college credits before they get their high school diplomas.

Fueling Academic Momentum

Advocates say dual enrollment works by creating academic momentum, especially for students who are otherwise unlikely to have gone to college. “Students are seeing themselves for the first time as college students. They’re seeing they can do something that maybe no one has ever told them they can do. It changes everything,” said Joni Swanson, who wrote a widely cited dissertation about dual enrollment and now oversees it for the Mount Vernon, Wash., School District.

Melinda Karp, assistant director of the Community College Research Center at Columbia University’s Teachers College, agrees: “These students might simply not have seen themselves as college bound. If you think of low-income, first-generation high school students, that may not necessarily be a message they’re receiving. Having a chance to take one or two classes in a relatively safe space and demonstrate to themselves that they can be successful is a relatively powerful thing.”

Advocates say benefits may also come about as a result of the partnership forged between staff at high schools and colleges as they design and coordinate course offerings, forcing each to communicate directly with the other about what preparation students have received and what they need in ways they haven’t done before. This may result in changes that better prepare students for the level of work expected of them in college, reducing the need to put them in remedial classes once they get there; decrease the time needed to earn a degree; and boost graduation rates on which public university and college budgets are increasingly being calculated.

Barriers to Future Growth

Despite the benefits, three problems pose challenges for educators and others seeking to expand dual enrollment opportunities, especially for underserved students: cost, quality, and obstacles to cashing in the credits.

Two-thirds of dual enrollment programs charge for these courses, according to a survey by the National Alliance of Concurrent Enrollment Partnerships (NACEP), which certifies the higher education institutions that offer dual enrollment. Courses can cost from $80 to $280 per student per credit hour, and while half the states provide funding for them, the other half let school districts charge students all or part of the tuition and fees and for textbooks, according to the Higher Learning Commission, one of the nation’s six accrediting agencies.

“If you’re coming from a low-income family, even a marginal fee of $50 or $100 could mean you may not be able to participate,” says Jason Taylor, a professor of educational leadership and policy at the University of Utah who found that high schoolers in Illinois can pay as much as $410 for a three-credit college course. To help lower financial barriers to dual enrollment, the Obama administration announced in January that it would look at ways to make federal aid available to students who would otherwise have to pay to take college classes while in high school.

Even if a student can afford to pay, if she subsequently chooses to go somewhere other than the partner college, there’s no guarantee the credits will be accepted toward a degree. Ninety-two percent of public universities and colleges give credit for at least some dual enrollment courses, a survey by the Western Interstate Commission for Higher Education found—about the same proportion as give AP credit. But many selective private colleges do not. Nearly a quarter of the dual enrollment programs that responded to the NACEP survey said they don’t even track whether their credits transfer.

Emerging concerns about access point to another challenge: finding high school teachers qualified to teach college-level courses, especially in urban high schools that serve low-income students. Along with the fees charged in some states, that may be why a study by the Illinois Education Research Council at Southern Illinois University at Edwardsville found that more students were enrolled in dual credit college courses in higher-income schools with larger enrollments of whites than in low-income schools with large proportions of racial minorities. There have been similar findings in Virginia and Pennsylvania. In Texas, Jobs for the Future reports, white students are overrepresented and other racial and ethnic groups underrepresented in dual enrollment compared with their proportion of enrollment. “Not all students have equal access to these equal-access opportunities,” notes Taylor.

Qualification requirements differ depending on the state and accrediting agency, but to teach a college course to high schoolers, a high school teacher generally needs a master’s degree in the subject of the course, or at least 15–18 credit hours in that subject, in addition to a high school teaching credential. “A lot of rural and low-income district teachers don’t” have those hours, says Taylor, unlike at elite schools such as Christian Heritage, where 63 percent of faculty members hold a master’s degree in their field. Even in well-funded public school districts it can be hard to find people with qualifications like those to teach, say, sociology, agrees Swanson.

“This is the next generation of work,” concedes Joel Vargas, vice president of Jobs for the Future. “Right now, in places that want to do more dual enrollment, they’re quickly realizing that they need to build up a core of teachers who are qualified to do this.”

Unanswered Questions

Other concerns remain, such as whether college-level courses taught to high school students by high school teachers in high school classes are of similar quality to those taught on a college campus.

An empirical answer to that question remains largely elusive. In a survey of Texas school administrators, AIR found that 87 percent thought dual enrollment courses were only as rigorous as, or even less rigorous than, AP courses. Researchers have yet to definitively compare the success rates of dual enrollment programs taught in high schools as opposed to college settings, says Karp. That doesn’t mean the dual enrollment courses taught by high school teachers aren’t as good as those on college campuses, she says, noting that in rural areas they’re often the only option, since there may not be colleges nearby.

There’s also the question of whether a student who earn enough credits to graduate with a bachelor’s degree at age 20 should be allowed to do it.

The Christian Heritage School, for example, will next year launch an even more intensive “early university,” through which juniors and seniors will take only college courses that will count toward bachelor’s degrees. But students who are selected to attend are being encouraged to make plans to earn a graduate degree right after getting their bachelor’s.

“There’s a caution to it,” says the head of school, Brian Modarelli. “We’re not of the opinion that it’s a good idea for a student to go on to college and get their bachelor’s degree in two years and, as a 19- or 20-year-old, say, ‘Hello world, here I am.’ ”

Jon Marcus is a freelance education writer based in Boston.