Here's a nice compendium of the application of the First Amendment to student speech. The author's conclusion:
Students possess First Amendment rights in the public school setting. However, there is much disagreement and a muddled legal morass as to just how much free-expression rights they possess. School officials obviously must ensure a safe learning environment and an environment that is conducive to education. Students must learn about the enduring values of a constitutional democracy, including the fundamental freedom of expression. If students do not learn and appreciate First Amendment values, there is a danger that these future leaders of the country will not protect those fragile freedoms in schools and elsewhere in society. As Justice Robert Jackson said about schools more than 60 years ago in West Virginia Board of Education v. Barnette (1943): “That they are educating the young for citizenship is reason for scrupulous protection of Constitutional freedoms of the individual, if we are not to strangle the free mind at its source and teach youth to discount important principles of our government as mere platitudes.”
There are certainly students who, intentionally or not, push the envelope and put the courts to the test in trying to define the boundaries of free speech in the school setting. I think it's practically impossible, especially given the available technology combined with the students' creativity. Nowadays it's almost as if students have to be told they can practice their right to free speech as long as they shut up.