@alex94: In 1969, there was a court case called Tinker v. Des Moines Independent School District regarding freedom of speech at school. This was based off of events in December of 1965.
In Des Moines, Iowa, students John Tinker, Mary Beth Tinker, and Chris Eckhardt wore black armbands to school, protesting the Vietnam War. School officials told them they must remove their armbands and the students refused. As a result of their refusal, they were suspended. Their parents got together and filed a lawsuit against the school district, claiming that their First Amendment rights had been violated.
The Supreme Court ruled in favor of the students. However, the Court did not grant unlimited rights to self-expression. The First Amendment must be balanced against the school’s need to keep order. It is considered acceptable as long as an expression or act does not disrupt classwork, school activities, or invade the rights of others.