It might surprise you to know (/s) that people in high school talk about sex. A lot. It's not unreasonable to assert that he does know roughly what's going on.
There isn't any way to accurately track how many people in a high school are having sex. We used to talk about sex a lot when I was in high school too, but I really have no idea how much actual sex was going on.
I guess you just need to write assertively and nobody will question it.