What's the point of writing a sequel if the whole thing is gonna be flashbacks to the time before the previous series ended?
I'd bet a shiny new nickel that this chapter and the last chapter will show up in the next tank before the chapter where they have sex. The author's afterword in Vol 4 indicates that there will be another volume, but there are only two stories left in the original run: Yamada goes to take her exam, and Yamada and Kase do the big nasty for the first time. Which is (A) not a lot of material for a tank; and (B) something you'd want as the climax of a tank, rather than a story you run into 1/3rd of the way through.
So I'm guessing you'll see this flashback and the previous flashback used to pad out the next volume in such a way that Kase and Yamada finally doing the do will be the end of the volume. Especially since this most recent chapter isn't even framed in the narrative as a flashback, so it can very naturally slot right in as the first chapter of the next volume.
And that makes a lot of sense. I was really wondering why on earth a sequel would start with 3 flash-backs.
This makes me kinda sad, though: I was really looking forward to seeing how their lives and relationship would develop through college.
Yeah, that would have been awesome... Oh well gotta be happy with what we get I guess.