In the vast majority of the nation’s schools, students are grouped by age and move along in lockstep from one yearlong course to the next until, at the end of the 12th grade, they acquire a sufficient number of credit hours—or Carnegie units—to graduate. It doesn’t matter whether they soak up the given material in September or barely grasp it in June; provided they get a passing grade, everybody gets promoted every year, all on the same schedule. Named after steel magnate Andrew Carnegie, the Carnegie unit (120 hours of seat time over a year) became the organizing principle of schools in the early 20th century, as the number of youth attending high school exploded and educators needed a standard way of tracking student progress in secondary and postsecondary education.

Slowly but surely, however, educators and policy makers are pushing back against the logic of promoting students solely on the basis of seat time or credit hours. Thanks to a burgeoning interest in proficiency-based education (sometimes called competency- or mastery-based learning), 42 states now permit schools and districts to award course credit and/or high school diplomas based on students’ ability to demonstrate mastery of key content and skills, regardless of the amount of time spent in the classroom.

Perhaps no state better illustrates this burgeoning movement than Maine, where, in 2012, state legislators passed a law requiring that by 2018 all of its high schools issue proficiency-based diplomas—a “certification,” as a Maine Department of Education official puts it, that students are proficient in district-defined standards and other skills. The law was the culmination of six years of work by the state department of education to identify a better way to prepare high school students for the next chapter of their lives. Maine educational officials organized proficiency-based education training across the state and underwrote pilots in several school districts.