I also applaud the OP for taking up his role of being a father. There are too many people in this world who just wants to be married and have kids, but do not own up to being a good father or mother. They just send their kids to school treating it as a free daycare. Being an active parent is somewhat of a lost art these days.
What strikes me as odd is that the same people who lament about young girls dressing too slutty and sexually promiscuous are the same people who complain about you being too protective, overbearing, etc. The fact is, kids need structure and discipline. They also need to be taught right from wrong. You cannot depend on the media to provide any of that. Filtering and censorship is a must. How much that is applied is up to the parents.
The first thing a young kid wants to be or act like is an adult. Their perception of what that is without parental guidance can be a scary matter. The consumer driven media, where sex sells and money talks, may not be the best influence for your kids. You can't blame it on one single source, ie a particular magazine, game, or TV show. But the collective combined message portrayed often influences what is socially acceptable by young adults and adolescents today. Why do think it's considered cool to be sexy at a very young age and losing ones virginity at that age becomes a major priority. It's tough being a good parent given all that's working against you: the media, the kid's peers, other parents.
Given the lifespan of a human being, a person has plenty of time (I do mean plenty) of being an adult. There's absolutely no reason for society to hurry them to adulthood. One should treasure their childhood. Good parents can help in ensuring that for their kids. I've seen enough people from conservative upbringings who turn up fine as adults. At the right age and aided by education they can deal with the real world. Sheltering them from an R rated movie at age 13 is not going to inhibit their adaptive ability.
Superself, if I were you I would send a polite note to the teacher that in the future you would like to be notified if your child is going to be exposed to inappropriate material prior to showing it in class. That gives you ample time to decide for yourself whether it is appropriate or not.