Automatically Turning Wifi Off when Sleep
SleepWatcher 2.2.1 can do this, and for free. However, if you do not have administrator access to your Mac (and it sounds like you might not) then you might not be able to install it. Worth a shot, but just FYI that it might not work.

Keyboard Maestro can do this, even without admin access, but it is not free. I think it's $35. Well worth it, IMO, because it can do 1,001 things on your Mac, and this is only 1.

I am not aware of any way to do this on macOS without using third-party apps.