If you see it all of the time, could please provide an example. Granted I don't follow HS sports very close, but I dont know many 8 man schools going UP a class.
Almost every skill you listed is different at 8 man. For example: coverage is almost exclusively man to man in 8 man. This makes all quarterbacking and passing different. Ie if you have a better athlete, they can get open.
Tackling, in 8 man if you break 2 tackles is an explosive play. In 11 man it's a first down. Blocking: it's way easier to wash 3 guys and bounce outside, vs getting outside in 11 man.
This is such an empty argument. "Yeah they all block and tackle and put their pads on just like us"...its significantly different. I'd like to see your examples.