Ending Corporal Punishment in Schools Act of 2021 This bill prohibits the Department of Education (ED) from providing funding to any educational agency or institution that allows its school personnel to inflict corporal punishment upon a student as a form of punishment or to modify undesirable behavior. First, the bill requires each state educational agency (SEA), in order to receive education funds, to submit to ED a state plan for eliminating the use of corporal punishment in schools. Additionally, the bill authorizes ED to award three-year grants to SEAs and, through them, subgrants to local educational agencies (LEAs) to assist them with improving school climate and culture. Grants must be used to implement school-wide positive behavioral intervention and support programs. Further, LEAs that receive subgrants must ensure that private school personnel are able to participate in grant activities. ED may also allocate funds to the Department of the Interior for schools operated or funded by Interior. Next, the bill directs ED to carry out a national assessment to determine compliance with the requirements of the bill and identify best practices for professional development and training programs. Finally, the bill gives protection and advocacy systems the authority to investigate, monitor, and enforce protections for students that are provided by the bill.