if it's based on the others' past, then we have Hyoutei and Rikka Dai Fuzoku.
In Hyoutei, it shows how the regulars we now all know met Atobe in the past when they just started middle school. We have Gakuto, Oshitari, Akutagawa (Jirou), Shishido and Atobe as the first years there and Ootori, Hiyoshi and Kabaji in their elementary days~
In the Rikkai part, we have the junior ace, Kirihara Akaya starting as a freshman there with the rest of the regulars as 秒 years in middle school.
