I was thinking the saaamee, it’s just so obvious
Also smiling like that so often, helping tidying up his messed up class schedule by personally talking to each prof.
Vaguely confirming what he can or can't/prefer to eat.
Its so obvious, full of hints
Word girl, you can read his face that he knows. Even if he's appearance changed drastically, anyone he spend most time on will notice. Idk, if the mc is oblivious or just straight up dumb
It's precisely because uke is feeling betrayed, that is why he felt awkward and don't want his teacher to know.
Even his sense of betrayal is mentioned several times in the novel.
And his apperance changed drastically, I guess it's normal of him to think that his teacher won't recognize him (he also thought his teacher didnt remember his name since back then he always got uke's name wrong).
Like, even irl, most people never really recognize big size people when they lost a lot of weight.
If I we're in his shoes i would straight ask him that I know that he knows I was his student. Because, being all that friendly to someone you just met? Even treating them to a not cheap meal? Come on now