Student Food Security Act of 2021 This bill expands the Supplemental Nutrition Assistance Program (SNAP) and establishes programs to address food insecurity among college students. Specifically, the bill permanently modifies the eligibility requirements to allow additional college students to receive SNAP benefits, and it establishes a demonstration program to allow college students to use SNAP benefits on college campuses instead of meal plans. The bill also advances implementation of certain data-sharing agreements and notifications to help students who apply for federal financial aid to also apply for and receive other federal means-tested benefits (e.g., Section 8 tenant-based housing assistance, Medicaid, and Supplemental Security Income). In addition, the bill (1) requires questions about food and housing insecurity to be incorporated into the National Postsecondary Student Aid Study, and (2) establishes a grant program to support research and address incidences of food and housing insecurity among college students.