It seems like he has been there since the show began even though we're now up to Season 6. And since the sixth season premiere episode was dated "September 26, 1980-something", that means that he is still in high school for the whole season. I mean, didn't last year's season finale and literally the episode before this one, revolve around Barry dealing with the end of his time in high school (pulling an epic senior prank and some really manufactured prom drama).

Granted, Barry has often been depicted as not being the sharpest tool in the shed and The Goldbergs has already been extremely lax with its timeline. But still, pretty soon, Adam is going to be in the same grade as him.

It means that Lainey is two years older than Barry. That’s a lot when you’re dating in high school.

I’m guessing it’s supposed to be the same school year as last season. Erica quit college her freshman year but would still be a freshman currently