I was under the impression that there was enough of a difference in schooling culturally between Europe and NA that a game written to be in the US would be noticeably different to what Europeans expect school is like
Yup, there is. The biggest one is probably more rigid lesson system, where the students don't just pick what classes they want to have as they please, but there's either a mandatory set of classes (in lower grades) or a few sets each with specific focus (e.g. science, languages, arts etc) a student can pick one from, typically in higher grades. As such, most students interact with the same group of people who attend the same class, than with different group from class to class.
Things like school-based sport teams or cheerleading are also very much US-specific.
If he was 19, the clunky scene with erryn verifying his age wouldnt have been necessary. Especially since he turned down the drink anyway.
TBH i don't even know what scene you are talking about, i don't remember anything like that. When does that happen, and are you sure Erynn isn't simply making fun of the MC because she
knows he isn't legally allowed to drink?