I guess I have to ask - why not a key lock?
My son is a senior in high school (sob!), and he has used a key lock on his locker ever since junior high. He cannot see the numbers on combination locks, and the school has actually provided him with a key lock throughout. I'm not sure why you wouldn't want to use a key lock, since they are not necessarily any easier to 'pick' than combination locks. My son puts his key on his house key chain that he carries in his pocket (or zipper part of his backpack). He hasn't had any issues with losing the keys, and the school also keeps one key in his file, just in case something happens.