it dawned on me overnight (forgot to add it here) that the reason the miyajo girls [2nd years] address tsukasa & rui [3rd years] with surname-san, even though the guys Are their seniors by age, it's because the girls don't attend kamikou. therefore the guys Aren't their seniors by proxy in a school setting.
which is why the girls address miyajo 3rd years with surname-senpai [airi, shizuku, mafuyu], because relative to their relationship within school grounds, there is an underclassman/upperclassman difference established.
the opposite is also true, with kamikou 2nd years addressing miyajo 3rd years by surname-san, like akito calling shizuku hinomori-san, airi is momoi-san, and so on
that was a bit of a clarifying side tangent but I feel it's necessary. I think a character's appellations is a fascinating subject, seeing how they view every acquaintance they interact with and how they decide what the best way to address them is very insightful :-)