As much as I completely agree with all of you that it was not right, I don't think the school should have been called over the tip. Behavior of the students (if it was an official school-sanctioned event) yes, but absolutely not over the tip. I always tip very well, but it is certainly my prerogative on what I tip, and even on whether I tip at all. I absolutely believe it is wrong not to tip for good service, but it is not a requirement.
While they were certainly not nice about leaving her a dollar, it is the customer's right to decide what to leave and unfortunately, the waitress doesn't get to say anything to the customer about it.
Please, as I know everyone is about to get angry with me, understand that I DO NOT condone this at all, and always, always leave good tips myself.