I am designing a new locker so it can fit my needs.
Before you put anything in them the lockers at my school include 3 hooks and a shelf at the top.
I would put a phone outlet, keep the three hooks, put more space for books, and a compartment for extra cloths or snacks, I would also put a trash can.
The outlet would be for in case you'd need to charge your phone in the middle of the day. I'd keep the hooks for my backpack and a hoodie.