That's my biggest worry, what have they been exposed to that makes them think this is a normal act between b/f and g/f at that age?
Ok, so when I was 13 a girl in my class was caught (by a few kids) doing this exact same with her bf. I am 40. 27yrs ago there was no internet, no mobile phones. etc.
They said they did it because they "wondered what would happen", they were both learning about their bodies and wanted to know how it felt.
There was no indication that either of them were subjected to anything untoward. It all went away fairly quickly, to be honest.
I guess what I am trying to say, is that teens are right in the middle of changing, figuring out how things work & there will be some experimenting. It doesn't always have to mean that they were subjected to anything.
OP, I would make sure you also reiterate that sex its a private thing, it isn't something to 'brag' to your mates about, or gossip about.