Every morning at 6 a.m., Charlie Skidmore, assistant headmaster at the Brighton (MA) High School, calls the city’s school department to find out how many teachers will be absent, and how many substitutes he’ll need to cover their classes. When there aren’t enough subs, teachers have to double up on classes, skip needed preparation time, and miss opportunities for professional development. "It puts a strain on everyone," says Skidmore. And not just at Brighton. Across the country, school districts are scrambling to find substitutes to fill empty classrooms.

While there are no national statistics showing just how many subs are needed, state data and anecdotal evidence confirm the problem. Several national trends are feeding the shortfall: ballooning student enrollments, dwindling ranks of teachers, and low unemployment that gives potential subs other options. "We were short of substitutes almost every day last year," reports Bob Minthorn, supervisor for school personnel in Hillsborough County, FL. "On our worst day, we needed 1,122 subs and I could only fill 914 of those vacancies. Substitutes who wanted to be teachers are long gone—they’ve been hired [as full-time teachers]."