What film is this about rebelling kids in a school with a ***** for a headmistess?

this film is about these kids in this school and the headmistress is really unfair its something about kids rebelling against her because this girl walked around school in a dress covered in condoms she did it to promote safe sex but the headmistress told everyone she was just encouraging sex?? its a good film it has a really good message but i can't think of the name ?