While kids are under school supervision, I can totally understand them monitoring what your children are doing.
If a child writes a Tweet from home, I can't see ANY way that the school could even THINK about expelling that child. But, it happened! Austin Carroll is a senior at an Indiana high school who got expelled (just before his graduation) for this (edited) Tweet:
"F*** is one of those F****** words you can F****** put anywhere in a F****** sentence and it still F****** makes sense."
Granted, not the most mature of Tweets that he could have posted, but that seems like the parent's area to deal with. Parents or school - who do you think should be in charge in this situation?