Filming & ages and grades of both Wally

What was the filming schedule for the last season? Was there actually any messing around with the school grades for Wally for him to be a Senior in high school? I couldn’t figure that was much of that for Wally.

Beaver’s season by season grades
Season 1 ‘57-‘58 3rd grade 7.75-8.5
Season 2 ‘58-‘59 4th grade 8.75 - 9.5
Season 3 ‘59-‘60 5th grade 9.75 - 10.5
Season 4 ‘60-‘61 6th grade 10.75- 11.5
Season 5 ‘61-‘62 7th grade 11.75- 12.5
Season 6 ‘62-‘63 8th grade 12.75- 13.5
Wally’s season by season grades
Season 1 ‘57-‘58 7th grade 12-13
Season 2 ‘58-‘59 8th grade 13-14
Season 3 ‘59-‘60 9th grade 14-15
Season 4 ‘60-‘61 10th grade 15-16
Season 5 ‘61-‘62 11th grade 16-17
Season 6 ‘62-‘63 12th grade 17-18

It seems that Beaver was supposed to be younger then Jerry actually was whereas, it seems that Wally was the same age as Tony Dow was. Jerry Mathers was born in June of 1948 so when the last show aired he was 15. He was probably going to be a Sophomore year in High School. Tony Dow was born in April of 1945 so the last show aired he was 18. He was starting college.