Personally I find the principle and the school were out of its jurisdiction in expelling this student. I would even say that the principle and the school are stalking their students.
How do we know that this principle is not some pedophile getting off on his students tweets?
What makes him or the school so morally righteous? Both the principle and school should be sued for invasion of privacy and stalking, what the students do on their own personal time at home or off school hours is not the school business period.
I also feel the same way about companies who interview their candidates and then do a back round check on Facebook and some other social network sites, again for me this is invading someone privacy and we the people should put a stop to it.
It is the parents job to discipline their own kids, granted some parents are too soft, but then again when you do have parents who are very strict their kids threaten the parents by calling child protection. So for the parents its damn if you do and damn if you don't. Enough with child protection laws and let the parents do their jobs as parents.
Coming from an Italian background my father was strict and when i was out of line it was the belt, as he often said "my house, my rules" "you don't like my rules then leave with the cloths you have on you" lesson learned. Heck he kicked me out of the house for not getting a haircut when told.