@Orcbolg no they were definitely dating during school, as evidenced by him referencing the extra chapter where she sexted him by "accident" while playing with a confiscated toy. He was bragging about it in class, and while he's talking about that, the Tachibana sisters put on a show for the serious faced guy.
Edit: the thing you're remembering was a sort of epilogue, probably because author thought it would be cancelled after one volume. Reminds me of one of Mitch Hedburg's funniest bits "I used to do drugs, I still do, but I use to too."