Instead of cracking the books, students at some New York schools are slipping in through the cracks of our immigration system, says U.S. Sen. Charles Schumer.

According to Schumer, several academic institutions aren’t schools at all, but fronts to hand out visas, allowing thousands of students to illegally enter the U.S.

Schumer asked the U.S. Immigration and Customs Enforcement to investigate what he labeled “sham universities.”

Three city schools were shut down in 2006 and 2007. They claimed to offer classes but instead only shelled out visas, said Schumer: Centurion Professional Training, Inc., in Brooklyn; Career Education International Group, Inc., in Manhattan; and GT Solution, Inc., also in Manhattan.

“These so-called schools are the equivalent of empty storefronts,” he said.

Because several of the 9/11 terrorists used student visas, Schumer called the issue a “glaring hole” in the city’s security.