Someone makes a gun locker that recesses in the wall between the studs. It is stackon style and you can put something in front of it and it is out of sight too.
Maybe $200?
A SCHOOL locker? You can get into those with a screwdriver or hammer. With a strong padlock, the handle fails, with a combo, you can bust it out. Those things don't keep out 16yo who have 5 minutes between class, what makes you think they are suitable for this?
You can get the stackon stuff, it isn't a safe, but it is better than nothing and stronger than that. It will only really work though if you have something that dispermits them from staying very long, like a suitable alarm system (which would cost a lot more than the safe unless you know what you are doing --but you can get a basic one to start with, just don't stop there).