I think madame_zora's got it totally correct, here. The students were informed of the parameters the school deemed appropriate for the evening in question, taking into consideration all the potential audience members, including much younger kids. In reliance upon this commitment by the students, the administration didn't offer warnings to parents so they could opt out, assuming they wouldn't be necessary (again, given the commitment).
At this point, however, the students realized what they, as young members of the aforementioned intelligentsia, were obligated to do. Faced with a ridiculously stupid administration and parents too provincial to have their opinions, beliefs and preferences respected, they decided to abandon their agreement/commitment and do what they wanted anyway, knowing full well it was better for everyone - including the youngsters - to hear the unedited excerpt. And further, that if the parents were given the opportunity to opt out, some would take it, thereby depriving the young ones of a wonderful opportunity and instructive evening. That anyone would call their actions into question at this late date is laughable. The problem was always with the administration for being so repressive and pussified in the first place, as madame_zora pointed out.
And yes, Sklar - the state can do a better job of raising your kids. You, as a single person, are susceptible to all manner of imperfections, as are all human beings. But collectively, we can obliterate the same. No more repressiveness and pussification. The only remaining hurdle of any significance is this old-fashioned idea that parents should be the principal authority in the raising of their offspring. As Hillary said, it takes a village - and ideally one that doesn't recognize illusory, and damaging, "parental rights."