Students in community college and other nonselective schools, unlike many of their counterparts at private colleges and top state schools, have to pay as much as 25 percent more if they want to enroll in 15 hours’ worth of classes. Since Pell students get no additional federal support, the 12-hour floor essentially becomes a ceiling. Few can pay without help so they stick with what the program allows, which essentially makes it impossible for them to graduate on time.

Here’s what the combination of Pell grants and per-credit pricing looks like in practice. Most Pell grant students are needy enough to qualify for the maximum annual grant of $5,645. Under a semester system, they would receive $2,822.50 each term. At an institution where that happens to be the tuition for 12 hours, students would pay nothing at all. But to take 15, they would have to come up with $705 on their own every semester.

In fact, tuition at community colleges is usually lower than the typical Pell grant. Students receive the difference as a refund. Since little institutional or state aid is available at these colleges, Pell refunds provide much-needed help for low-income students, who are often older or working, to defray the cost of books and living expenses.

But when Pell grants are capped and tuition isn’t, a student taking 15 hours ends up getting a smaller refund check than one who takes 12. A student enrolled in 12 hours at a community college charging the national average of $100 per credit hour would receive a refund of $1,622.50, while one enrolled in 15 hours would get $300 less. In effect, these students are being paid not to take a full schedule. A recent survey by Complete College America and Postsecondary Analytics found that less than a third of technically full-time students at two-year colleges actually took 15 or more hours.

Congress could solve this problem by awarding larger grants to students who enrolled for more than 12 credit hours. This would change how students and institutions think of full-time enrollment and would probably increase the number of students who completed degrees on time. Since students would need grants for fewer semesters, the government could also increase the size of annual awards without increasing spending.