This is a tough issue.
First of all, as we covered earlier, minors don't have rights in this country the same as adults do. So, first ammendment is kind of out the window right off the bat.
A nude, depicting a studen, even a self-portrait is considered Child Porn. I can't do a nude of someone under 18. Well, I guess I CAN, but I can also get in trouble for it. There are some exceptions. I guess I could do as much as they show in diaper commercials. But, even then, people can throw a fit and I could wind up in court.
The school doesn't have the right to delve into students sketch books and say what they can and can't draw, but they DO have the right to decide what will and will not be representing their school.
Just as eBay can regulate what I do and do not list.
Art is subjective and as Rogue Hunter has pointed out, not everyone cares to see past the surface and look for the meaning behind it. Personally, I'm intrigued by a Crucifix in a jar of urine and can imagine LOTS of meaning behind it. Could there be a more "universal" way to get the point across....probably. I have tons of ideas every week that I have to "amend" to be more socially acceptable. On the other hand, another method might not have been a shocking and to the point and perhaps that's what the artist was wanting.
It's a fine line and an artist of any means has to decide which side of it they want to be on. If you are in a punk rock band and wish to play "shocking" music, it's going to be slapped with a parental advisory and not played on the radio. If you pee on a crucifix, you may get your point across to a few people, on the other hand, lots of people aren't going to bother to try and see the meaning behind it.
Me? I'm go more "mainstream" because my intent is to actually make a living. I try to keep my ideas as vague as possible so that they mean something to a larger group of people.
It stinks, in a way. Because children should be allowed to express their feelings through art. But, just like the rest of us, they have to remember that when you go out of the mainstream, 1.) the place you intend to display the art can easily say, "Um....nope. We don't want this representing us" and 2.) that is won't be as universally accepted or even attempted to be understood.