If had you're bike registered, you would've had it back by now...but i'm guessing you didn't, so that doesn't matter.
The best advice I could give you would be to keep trying with the police and/or you're parents. Unless you're bike was stolen from school property, there is nothing they can do (did you leave it on a bike rack at school? if so, maybe they have a security camera you can look at.
To be honest, I would've stolen it back by now, but that's me. You're best shot at getting it back would be to keep trying with the cops. If that doesn't work, you could just get a new bike and register it! Hope this helps.
Oops, i misread you're question.....sorry. Have you tried talking to that person, in a calm, friendly manner. Tell them that you really liked the bike and would consider it a personal favor if you told you who took the bike. But keep trying though!
