A public interest group says as many as 900 colleges are pushing students into using payment cards that carry hefty costs, sometimes even to access students' financial aid money.

The U.S. Public Interest Research Group Higher Education Fund says in a report published Wednesday that colleges and banks often make secretive deals allowing them to rake in millions from the fees. The report says some of the fees appear to violate federal law.

The report says more than 9 million people, or two out of five U.S. higher-education students, attend schools that have deals with financial companies.

The fees add to the mountain of debt many higher-education students must take on. U.S. student debt tops $1 trillion, and has surpassed credit cards as the biggest source of unsecured debt.