Get a LAWYER

You did give consent to let them search your phone, so going the legal way immediately would both be a good and bad decision. I'd say ask them to give it back and if they refuse, lawyer would be the way to go. They broke the law if they had a child interrogated without an adult/legal guardian or a lawyer in the room. Either way you go, good luck on the outcome! I'm sure you'll get it sorted soon. Try giving them the idea of tracing the email. I'm sure they might've already tried, but just try to prove your innocence as much as possible. Going the lawyer way could mean paying big bucks $$$$$, so try to settle out of court. Best option both for time and financially.