In my experience, kids usually learn or first hear cuss words on TV or in movies (viewed at home), then they "try them out" with their friends at school or hanging out.
As I have said several times - I can go along with the peer group issue, I've experienced it with my daughter - but her "peer" learned it from her parents. and I know this because the parents are friends of ours and have a tendency to forget they are not on a construction job site at times the children are around.
However, peers are NOT the school. If more children were taught by their parents NOT to use those words the amount of peer pressure to do so would decrease.
There is a minority of foul language learned at home from parents that are fools and teach it to their children.
I was a teenager before I ever uttered the word for excrement - but that was only after having heard it from my father after he dropped a brick on his foot.