For me it depended somewhat on the subject, for some I'd get the equivalent of 10/10 with very little effort, for others it would have required somewhat more work.

Even if I'd literally gotten perfect grades in every subject, though, it still wouldn't have told me that I was the smartest kid in the school. Since I never bothered asking others for their grades for the sake of comparing them, for all I knew there could've been twenty other kids with equally good grades.

Also, getting good grades only told me that I was good at school / playing the system, and I had serious doubts of how well that translated into "real-world" intelligence.