Protecting Our Students and Taxpayers Act of 2021 or the POST Act of 2021 This bill requires a for-profit institution of higher education (IHE), in order to participate in federal student-aid programs, to derive at least 15% of its revenue from sources other than federal funds. Current law requires a for-profit IHE to derive at least 10% of its revenue from sources other than federal funds. The bill also expands the definition of federal funds to include GI education benefits, with an exception for the monthly housing stipend provided under the Post-9/11 Educational Assistance Program.