Just establishing the timeline here, ignore me.
Season 1 takes place in Fall-Winter 1983, making it the 83-84 school year.
Season 2 takes place in Fall 1984, making it the 84-85 school year.
Season 3 takes place during Summer Break of 1985, making it between the 84-85 and 85-86 school years.
Season 4 takes place in Spring of 1986, making it the 85-86 school year and also Spring Break 1986.
Steve is a grade higher than Nancy, Robin, Jonathan, Barb, Crissy, Jason, and Billy. He graduated in 1985, making his senior year the 84-85 school year. He was a Junior in in 83-84 school year and the rest were Sophomores. Tommy and Carol, along with her friends, are presumably in the same grade as him. Barb is presumably in the same grade as Nancy. Billy is implied to have been a grade below Steve. Given that Chrissy is presumably a senior in the 85-86 school year, she would have also been a Sophomore in Season 1. We can assume that Jason (at the very least) is in her grade as well.
Eddie has repeated Senior year twice, meaning he's in his third Senior year during the 85-86 school year. That means his second Senior year (and first repeat) was in the 84-85 school year, and his first Senior year was in 83-84. So, he was in his first senior year in Season 1. He's a grade higher than Steve and two higher than Chrissy.
Eddie participated in a middle school talent show, which Chrissy also participated in, obviously meaning they both were in middle school at the same time during this talent show. That would put this talent show in the 79-80 school year, when he was in 8th grade and Chrissy (as well as all the others in her grade) was in 6th. Steve would've been in 7th grade.
Assuming that Gareth is a Junior, that would make him a Freshman during the 83-84 school year. That also puts him in 5th grade when Eddie is in 8th. Unnamed and Jeff are likely somewhere between.
This is all also assuming the only one to ever get held back was Eddie, and only just the twice that he was held back as a Senior, nothing more and in no other grades.
The younger kids are all presumed to have been in 7th grade during the 83-84 school year, none of which were ever held back. Erica is likely to have been in 6th grade during the 85-86 school year, putting her in 4th grade in Season 1.
Anyways, it was food for thought. Also helps me remember. This is also the most likely timeline that I can think of. Let me know if i missed/forgot something.