It doesn't belong in ANY school. Not this issue that's for sure. My mom and dad made sure they were the providers of this kind of teaching. They would ALWAYS preview the material if there was a sex ed program. Sometimes we were kept out of it, sometimes not. When we were allowed to go through the class, there were both abstinence and non-absitinence based ones. It wasn't a big deal because we had been taught properly by our parents and we were strong in our values. We (all eight children) were able to deal with things in the very few times there were conflicts. Not one of us ever had pre-marital sex.
For the fifth graders here, it is done AFTER school for one day, boys and girls being separate and the students attending with parents. The class is not taught by any teachers or school personnel. It has never been much of an issue.