Totally thought they were in middle school. Definitely on the older side of middle school, but no way would I have thought they were the equivalent of juniors in US highschool.
@Acolytus Remember last chapter he ran into Kubo-san's sister at the mall by the lingerie store, and his little brother Seita gave away that he liked aqua over pink. Seeing that Kubo-san bought an aqua-colored bra after that tells him she bought it based on his color preferences.
I think it's fair for that to override armpits in this case.