do not tell anyone your password and change it often if you give your password to someone they can get into most of your accounts and change lots of things.
don't reply to the messages because if your mean aswell your get in just as much trouble when it gets found out.
The consequences our big at school you could get a detention or a isolation and it is breaking the law so the police could take action against you.
save the messages
make sure you save the mean messages and then show someone the messages.