In episode 1 of season 2 Beaver says he is in the 3rd grade. So, according to that he should be in these grades-

Season
2 - 3rd
3 - 4th
4 - 5th
5 - 6th
6 - 7th

-but didn't he graduate from the 8th grade in season 6? :confused:

Yes, Beaver is an 8th grader in season 6, so he really should have been a 3rd grader in season 1, not season 2.

With regards to Wally, he's a 12th grader in season 6, and said to be an 8th grader in season 1, when he really should have been a 7th grader.

In episode 1 of season 2 Beaver says he is in the 3rd grade. So, according to that he should be in these grades-

Season
2 - 3rd
3 - 4th
4 - 5th
5 - 6th
6 - 7th

-but didn't he graduate from the 8th grade in season 6? :confused:

Yes, Beaver was not in the 7th grade in any season.

Being born in June, Mathers himself would have been young in his real grade. So in the show he would have looked young in his "real' grade, which SHOULD have been:

Season

1 - 3rd
2 - 4th
3 - 5th
4 - 6th
5 - 7th (growth spurt would have made him look "correct" in this grade)
6 - 8th (the show "trued him up" in this season)

Wally was screwed up the other way. In season 1 he should have been in the 7th grade.

All of this is just what 1960s said, only he said it better.