If you are a US passport holder, you do not need a visa if you are studying for a semester in England; however, you will need a valid U.S. passport.
Changes can happen anytime. Check the UK government site to see if you need a visa.
It is the student’s responsibility to check for updated visa procedures and required documentation. If you have any questions, your best resource will be your host university or the consulate.
NOTE: Visa application rules change frequently and the university cannot control visa acceptance. You are responsible for applying for and providing the proper documentation to the consulate or embassy.