hmm, so there are NO limits on what she can say, simply because she is enrolled in a public school? So she can feel free to publish slanderous accusations about the principal's sex life, and the school will be powerless to react? Interesting.<quoted text>
If the school were a private school, or if this incident happened at a private company, I would agree with you. The clinker here is that this is a public (i.e., government) school, and that's when the First Amendment kicks in.
If a private employer wants to limit your speech, the First Amendment has NOTHING to do with it, and nothing to say about it. But the government -- well, that's a different story. The First Amendment prohibits the government from limiting free speech (within the "shouting 'fire' in a crowded theater" limits established by the courts).
Insofar as this public school is an arm of the government, as it doubtless is, it must grant this student her First Amendment rights even if they loathe what she said or wrote.
(Don't tell me that my hypothetical rises to the level of yelling "fire" in a crowded theatre, because it certainly doesn't, that is why I made up this kind of example).