Canadian public school here too. Depended on the teachers. I had some that did this all the time. Others that never did it. Teacher would collect all the papers and then pass them back out again rather than "pass to your neighbour" or whatever, so it was very unlikely to get the paper of one of your friends.
Never changed an answer for someone else. I don't think that it would have ever crossed my mine to change an answer for someone else. Why would I do that?
I think I've only been in one class where we would grade each others' tests and homework, but because the work we'd be grading is usually by the person seating in front or behind us and because we were friends, we'd usually "give ourselves pats on our backs" as it were.