Karamatsu and Ichimatsu probably did spend most of their highschool together.
As everyone reminisces about their highschool days, Karamatsu and Ichimatsu's conversation seems pretty isolated in the corner over there.
They probably would have a lot of things to talk about because of how much of their memories involve each other.
That's why when it comes to talking about highschool, Karamatsu chooses Ichimatsu to have this conversation with. (Except that it seems like Karamatsu is the only one more willing to reminisce about their past together than Ichimatsu is.)
Like the time him, Ichimatsu, and Jyushimatsu pranked Osomatsu by stealing his pants.
Or when they were teamed up during the festival.
Or when Ichimatsu and Todomatsu blatantly used him as a fort in a snowball fight.
Or how they used to always eat lunch together on the school stairwell.