Honestly I agree with you. The characters are fine and cute but it's totally obvious that the author just wanted to make a story with the "teacher x student" trope but trying to make it legal, so of course the context could only be forced... First of all it's clear that the coma was just used as a pretext to make him return to school as an adult, but the point is that this just makes no sense, because you can't have an adult of that age (22/23) who enroll in high school, like that, after years, it was just weird! Imagine if he woke up at 30, or collapsed when he was in middle school, then would he still have been allowed to re-enroll? Of course I'm exaggerating, but that's the logic, apparently.
Oh no, of course everyone has the right to graduate if they want to resume their studies, I didn't mean like that! But for example in my country there are specific courses to attend for adults who didn't graduate at the "right" age and want to get their diploma, but of course you can't be like 40 years old and enroll in high school or in middle school... Only university allows enrollment to people of every ages (but you need to have the high school diploma of course), so this situation was just really weird to me But yes, I agree with all the other things that you mentioned!
Now that I think about it you're right too, eventually he's just 3-4 years older than his classmates in this case, but it didn't help the fact that the professor is the same age as him! Indeed, my disappointment comes from the fact that the whole situation is actually quite bizarre... I don't know but it felt really weird to me that they were childhood best friends and school mates and now they've reunited and started dating in this student-teacher dynamics... It's just my opinion but I think at this point it would've been much better if they were complete strangers to each other, and he still could've been like 22-23 but the professor needed to be a few years older! I might have liked something like this.
This made no sense