In what circumstances is the right of free expression in schools not absolute?

Answer 1

Freedom of speech is not absolute anywhere. One person's "freedoms" end where another person's freedoms begin.

Check this site:
http://en.wikipedia.org/wiki/Freedom_of_speech

This site specifically talks about freedom of speech in school:

http://www.freedomforum.org/packages/first/curricula/educationforfreedom/supportpages/L08-CaseSummaryTinker.htm