This issue also bears similarity to airport security after 9/11. America's becoming an expert on inconveniencing the many based on the actions of a few.
Students in public schools have the right to free speech, so long as it doesn't interfere with the operations of the school.
http://en.wikipedia.org/wiki/Hazelwood_v._Kuhlmeier http://en.wikipedia.org/wiki/Morse_v._Frederick
I consider these decisions disgusting, wrong, and anathema to the Constitution, but there you are.
I think this is mostly because in grade schools students aren't legally considered adults. If I was being really cynical, I could say that the only reason there aren't 'student rights' in grade schools is because grade-schoolers don't vote.