Exactly...on their property. But not off their property.
But as i rethought later on this thread, it is also a private school's perogative to stipuate the criteria for their students, which may specify that students meet certain GPAs, write convincing essays, or not have any internet webpage connections.
I believe most Christian parents would not want their children using myspace because of the pedophiles and other sex offenders who can easily get on themselves. Trouble is, lots of parents, christian or otherwise, don't have a clue as to what their kids do in the privacy of their rooms.