...However, dear parents, your bright idea of pulling your kids from class? First of all, unless your kids are the SOCIAL PARIAH of class, they will learn what it was that they were supposed to have learned from the teacher, except as taught by their peers. With color commentary, probably with rude slang, and really, you don't want to see a sixth-grader's idea of an anatomically correct drawing of genitalia.
Pulling from class, unless it's for an unambiguously positive reason (like gifted & talented class) is an embarrassment. It singles you out from the other kids, and unless you have the kids sufficiently under your thumb that they will believe what you want them to believe without question (which you're unlikely to do if they're around their peer group and have been learning about the many ways that parents lie to you) they may not view being singled out and not learning something as a good thing. If they're academically lazy, they may like the idea. If they have inspiring teachers, and the rest of the kids are doing something that they're excluded from, they'll probably be feeling humiliated and cheated out of an enjoyable learning experience.
When they come back to the class, they are going to ask their friends what they missed. Their friends are going to tell them. If it's something that they consider not worth that much parental reaction, they're going to be wondering what the big deal is. I know if my parents barred me from learning something in school, I would not feel comfortable asking them about that topic, because they would have demonstrated to me that they were not comfortable with the idea of me knowing this. That would discourage me from asking them about it. I'd ask my friends, maybe my teacher, and might poke around in the library.
If you're going to teach your kids your way vs. the way that school plans to teach them, for goodness sake, do it beforehand. That way the kid will have already been exposed to the topic, and their first impression will be of your presentation of it, rather than whatever the fuck their classmate says.