While I do agree that it is not good for these kids to be viewing porn sites, I think you need to realize also that they are high school students, not elementary or even middle school. The school is right in banning access to the porn sites, but do not for one second think that, even with schools banning it and parents keeping a watchful eye, these kids will not find a way to view such material somehow, somewhere. Teenagers can be pretty darn crafty.
Additionally, and once again I don't think this behavior should be encouraged, but in many cases these kids are going to be on their own, either working or in college, in less than a year. They will have complete access to the materials if they so choose. Now, once again, I'm not saying the school is doing anything wrong. But, I'm not sure they need "professional intervention" for wanting to see a bit of pornography either.
Sheesh, putting filters on wouldnt really help anything. Some computer savvy highschooler would just disable it within a week.. heck, I know if I was still in high school, I would have done my best to try (Not because I wanted to look at porn in school [Whats the point of that really aside from snickering about it with your friends?], but rather just to stick it in the eye of the administration ;)