How do I reject someone I know and can't avoid?

Straight to the point: I (19F) been hanging around with different friend groups in school since I started last fall, and one of the groups that consists of me and mainly male students. Our talking topics are usually very sexual oriented, but I haven't minded it 'cause I like that kind of humor as well, and it's not like we talk about personal stuff.

Anyways, I think one of them has started to see me differently or something, because I just got a text out of the blue saying "you're nice :)", and my gut is telling me that it's either a confession or he's gonna confess.

The thing is that I have no interest in this person. Also I don't know everyones ages in school, but I'm pretty sure he's at least around 15 years older (which is already way past my limit). But I also cannot ghost him 'cause we have same courses and the groups in them are really small, so no way of avoiding conversations in school.

I guess my question is: how can I clearly put him in the friend zone so that we can continue to coexist and to be able to study in the same groups in the future? And I also don't need him to get any closer to me than he is now.

I think I could manage face to face, but I have no idea how to do so over a text, because I'm not sure what exactly is he thinking when sending that text.

So any appropriate advice is welcomed. I'd never thought that I'd be asking this on reddit, but all my friends are from school so I have no other way of asking anonymously.