Fuck was definitely pretty standard in the playground when I was a kid, certainly in yr6, probably less accepted by the other children in yr5 though it was used. BUT never in front of teachers or parents of course.
I also remember getting a pretty big bollocking for calling another child an ignoramus and getting another child into trouble because he called me a cow. In yr4 I only felt one of these was a bad word, and it wasn't the one I used!
The wee lad I look after came home from P1 the other day with a new word, "scanky", though he was referring to the bin so really he wasn't that far off but I was still a bit
at that word coming out of the 4 year old's mouth!