If you have contact tracing enabled within the app and your phone is close to others in adjacent lockers it will record you as being in close contact.
When you store your phone in a locker or other communal area on campus you should pause contact tracing within the app by moving the contact tracing toggle.
Remember to turn contact tracing back on afterwards. When you pause contact tracing in the app it will give you the option to set a reminder to turn it back on after a certain period of time.
For more see the NHS COVID app FAQs.