If I was the cop, I would have shot BITE_ME. Or sent him to a Federal PMITA prison. (in prison, smart asses get sodomized)
But seriously, the key to most situations is keeping a cool head. Keep your facts straight and you won't contradict yourself. (For instance, if you had one key, but not all of them, say that fully - try not to have to correct yourself) If you don't contradict yourself, you're a less attractive suspect. Also, if you're a minor, you have less to worry about because most penalties are significantly reduced for minors.
Remember that the cop is doing his job - I doubt he's got anything in particular against you. If you look at the balance of things, against you there are two statements, one of which (your friend's) you can weaken by its circumstances. That leaves one unimpeached statement, and one doubtful. On your side, you have the fact that the teacher has a tendency to lose these keys - certainly enough to explain their absence if they're missing, so an offhand "He probably just lost them again." can raise that question. Also, if your own keys are similar in appearance to the missing ones, that can bring question onto both statements. ("My own keys look somewhat similar to the chorus teacher's keys, maybe this person mistook mine for his.")
Right now, I'd say that he has nothing for a criminal charge and very little toward a civil charge or school disciplinary hearing. Just think logically about things - the cop's credibility doesn't really enter into the matter because he isn't a witness. So, with no
guaranteed credible witness, and no evidence, there's nothing serious against you.
It's actually possible that nobody has fingered you specifically, and he's just trying to see who he can rattle into confessing. Remember that you didn't finger your friend, yet the cop said that you did? Well, anyone who would lie about you would lie to you. It could be that you weren't specifically fingered either, or that he doesn't completely believe his source and is putting pressure on you to see if his source is telling the truth.
Basically, the short version is, it doesn't sound like you've got too much to worry about yet as far as charges being filed. As long as you keep a cool head and don't stand out too much, you should be fine. Unless, of course, you're a minority - in which case you're likely to be shot.