DEny deny deny. That may have been your pipe but you lost it at a party last weekend and have NO idea how it ended up at school.
Get a lawyer.
Now... what have we learned about weed in school? NOT PRACTICAL. You're gonna get fucked one way or another, and it's a huge distraction when you are trying to get your diploma and go on to bigger and better things.
The worse you fuck yourself when you are young, the longer it takes to dig out of that hole. Your parents must be really upset but it is very good of them to take the approach that the school should not punish you without hard physical evidence. Make sure you let them know how much that means to you; even if they end up grounding you until your 40th birthday, at least they are being reasonable about it.
Good luck.