Help with horrible tank setup

This yellow-bellied slider has been a class pet for years while being kept in the same bare tank with just one free-floating dock. Recently, I volunteered to help maintain the tank. I’ll be in charge of feeding, water changes, and general cleaning and maintenance.

I know that this tank is far too small for the animal, but as a highschool senior with very little time and money to pour into this, purchasing a new tank myself isn’t an option at the moment. I have reached out to the school itself to request some way to get a more appropriately sized tank for this guy.

I’ll gladly take literally any advice from more experienced turtle keepers, though I am trying to focus on stuff to generally improve quality of life.

This animal is not my property, and offering to take it would be irresponsible of me, since I will be moving out soon. So there may not be much I can personally do, but I really would like to make the best of a bad situation.

I’m planning to add some gravel/sand, small bits of hardscape (not enough to take away swimming room), fake plants, and a more secure dock for the animal to rest on. Product recommendations and additional advice are greatly appreciated.