I have two daughters in the equivalent of Y6 and Y4 (we live abroad). Both are bright, happy, sociable kids. Their first few years at school they were getting "top marks" in everything - though the assessments were pretty gentle. Now they do more and more tests and for both of them their marks are dropping to "average".
I coasted as a kid. I was bright and could get away with the minimum, so never really had to put the big effort in. I was always relieved to not get crap marks, and "happy" with my mediocre marks, but always a little ashamed that I didn't do better.
I've said to both girls that really I don't care about the marks themselves, but do care about a) how much effort they put in and b) how they feel afterwards (ie proud rather than relieved/ashamed).
But how do you reinforce that in them? They are so capable, and I HATE to see them go down the same path I did. It made me shy away from challenging opportunities, and whilst I've done ok now, I do know I could've made choices that made me feel good about myself instead of blocking out certain opportunities because I was scared they were going to just prove that I should've worked harder.
They both do some nice extracurricular activities where they see the obvious rewards from putting in the effort... It just doesn't end up the same when it comes to school work...