Depending on your area you can call kids help phone or crime stoppers. Crime stoppers is the best of the two and it will keep you anonymous. the website for crime stoppers is http://www.crimestoppers.org/
Kids help phone:http://www.kidshelpphone.ca/en/home.asp?gclid=CILJzpHdqpUCFR8SQQodoRcokw
I suggest telling your principle. You are not a rat for doing that. Ask him/her to keep you anonymous and he/she should do so.
I hope that helps!