I have to agree with Dido on this one. What can be done is write the school en masse, and all the bodies that go above it. It needs to be made VERY clear to parents that they agree... err.. halt that thought.. something else popped in.
When does a child get sex ed? You will see penises and vaginas then! If that isn't porn compared to the (in general) modest art, I don't know what is

But yes, parents should have to sign an agreement that THEY are responsible for checking out where the children are being taken, and that they can pro-*test*-('") BEFORE the event takes place, thus enabling them to not let their child go along on said trip, and the child will be in school attending (if need be) another class for that time period. After all, trips are usually announced with plenty of time to spare.
Just wait and see how much whining they will get from the kiddo, all the friends are coming along and he can't! Plus that it puts the schools blame free, since the 'rents WERE notified up front. It shouldn't be the responsibility of the school to check if there are possible offending things to be seen..
Crazy thought: we cannot take a kid to the zoo either.. I mean.. ever seen a chimp play with you know what? Hrmm.. that's IT! No more zoo trips either.