They load up on course credits early (six in summer 2 session, 12 in fall semester, and maybe six cushy ones in winter intersession), so they just barely have enough to be in good academic standing when they bail in the spring. You need 24 credits in one academic year to still be eligible and not cost a point.
The problem comes when players don't have the grand plan in mind from the beginning. Turning pro early most of the time means withdrawing from class and doing the workout/agent circuit. If you've known you were going to only play a year, you take the easiest credits you can get and don't cost your school a point as part of the deal. If you blow up as a soph or junior, and don't finish that spring semester, you may be short (or seniors do it too).
One advantage football has at I-A - practically everyone redshirts their first year, so they get nine semesters in before combine time, instead of seven.
