He's 60 or 70-80? When my father was a boy in NYC in the 1920's - 30's he skipped two grades and then went to Stuyvesant High School.
I would double check the details of his education with him - he may have skipped grades.
He's in his mid 60s, so I guess that he was educated 45-55 yrs ago for the most part. My grandmother, born in the early 20s, had a similar experience to your father: she skipped two grades and graduated high school at 15.
My understanding from my mother (they married when I was an adult), is that he grew up in a very small rural area, didn't do particularly fabulously in early school and was sent off to military school or boarding school of some sort around high school. I am sure that he didn't skip any grades, though.
