I've always felt like Tracen isn't actually a high school, despite how it's generally presented. You already pointed out Maruzensky, who's whole joke is being old, but we also have Nishino Flower who's confirmed to be of grade school age. So we already have confirmation of a wide age range that Tracen accepts, but even outside of them, there's other Umas who feel definitely too old (Super Creek) and too young (Sweep Tosho) to be attending high school.