Thousands of prisoners throughout the United States get their college degrees behind bars, most of them paid for by the federal Pell Grant program, which offers the neediest undergraduates tuition aid that they don’t have to repay.

That program is about to expand exponentially next month, giving about 30,000 more students behind bars some $130 million in financial aid per year.
The new rules, which overturn a 1994 ban on Pell …