I think you're wrong. My (admittedly more or less adult) friends go through a remarkable language transformation when their parents or other authority figures are around. And I rarely hear kids with parents in earshot using the major baddies, at least. It's always when they're just with each other.
I wouldn't necessarily blame the schools (although I wish they'd use the soap treatment!) but negative peer pressure.
Peer pressure I can go along with.............but somewhere along the line they had to have learned these words from somewhere.........and that somewhere is usually an adult.
The utterance of those type words, including the word "damn" elicit a financial penalty in this house.........my 6 year old tends to heavily supplement her allowance when daddy is doing yard or auto work!!!!!