The kids are taught sex ed after they are seen examining a dog's genitalia... however, the girls and boys are taught separately, and by the most incompetent teachers imaginable. The boys essentially learn nothing, and the girls learn that if a boy isn't wearing a condom, they could get sick... ignoring the actually having sex part. When the boys go to buy condoms so the girls will talk to them again, the parents find out, and extend sex ed even to earlier grades.
So, in other words, a lot like sex ed in the real world? :-)
That ep was hilarious! I lost it when Cartman was peeing inside the condom and throwing it into the trashcan busting it open like a water balloon.