This is a cute story, but it does give me the creeps when the teachers start talking to each other. It’s rather predatory and gross if you think about it too much (like so many shoujo tropes...). I also don’t see why the MCs need to act mature all the time, or why their view of maturity means they can’t date. Wouldn’t having an SO be mature?
To address an earlier comment, I think actually we tend to culturally ignore male student/female teacher relationships because there’s an attitude of “way to go” to the boy for getting an older woman, whereas girls we see more as victims of grooming. It’s a double standard for sure and there are problems either way. A 10 year difference isn’t a big deal as you get older, but at 14 it’s huge. Just consider yourself at those ages and imagine if an adult would want to date you. Would 24 year old you want to date 14 year old you? I hope not. Sure, the ages are arbitrary and it won’t be the same for everyone, but generally speaking you’re doing a LOT of growing mentally and physically in your teens to early 20s.
Anyway, I recommend just enjoying the characters, without thinking about their ages too much. I do wonder if Nagai will really go into music since he doesn’t actually seem to enjoy it particularly, nor have the drive to be competitive.