Any changes made to a page like this, such as the purpose, intent, and other such variables, is posted by Facebook and the record of the change is readily available. I run a page for the forum I run, so I am rather familiar with it all.
As Ann said, it requires action to become a fan. You have to accept the invitation or search out the group/page and click it. It then posts your your Wall and to all your friends that you joined the group/page. It is not something that can be done by accident.
I am in support of the school for this on the grounds that it was an attack on the teacher. Free speech outside of school is fine and I totally support that... but this was a case of slander and libel and should have been acted against no matter who was the target. the student(s) took school activity outside of the school thereby involving the school in it and the school has taken the appropriate actions. Expulsion would have been an option as well, but the school didn't go that way with it.