K-12 Cybersecurity Act of 2021 This bill requires the Cybersecurity and Infrastructure Security Agency (CISA) to study the cybersecurity risks facing elementary and secondary schools and develop recommendations that include cybersecurity guidelines designed to assist schools in facing those risks. The use of such recommendations shall be voluntary. The study must evaluate the challenges that schools face in securing (1) information systems owned, leased, or relied upon by those schools; and (2) sensitive student and employee records. Further, the bill requires CISA to (1) develop an online training toolkit designed for school officials; and (2) make available on the Department of Homeland Security website the study's findings, the cybersecurity guidelines, and the toolkit.