Nonsense. Private schools can have any rules, and any conditions for remaining, that they like. Parents agree to the rules when they enroll their kids, and generally agree to abide by whatever rules may be imposed/changed from time to time. If the parents don't approve of the rules, they can take their kid out of the school.
There's a very legitimate reason for this. Parents who are forking out money for private school are doing so largely to ensure that their children have a peer group to grow up with, which is being raised according to values similar to their own. I wouldn't want my kid hanging out with kids who are involved in the sorts of things commonly going on on MySpace.
A few years back, a very selective private school in my area reacted to the shooting death of one of their (low-income, black, scholarship) students at the hands of a relative, by announcing that beginning with the following school year, they would require all parents to sign a statement representing that they would not have any guns in their homes. It was perfectly legal for the school to do this. It was also perfectly legal for a sufficient number of parents to bluntly inform the school that their child(ren) wouldn't be returning the following year if the rule actually went into effect. Last I heard, the proposed rule had been quietly withdrawn, and the school's administrators had presumably absorbed their surprising discovery that not all "good people" think what they think about guns.