Does the 1st amendment protect my personal privacy?

[gentle giant 2]

You have no first amendment privacy rights. No one has.
The US Constitution has no guarantee of privacy. In fact the word is never mentioned.
If you post something on Facebook it becomes public property and anyone can see it. I doubt, however, that the principal has any right to take actiuon for something which you did on Facebook unless you made overt threats or slandered someone.
It is not good practice to post anything which you want to keep private, on any social networking site.