@Pomoek I looked it up in raws and Fushi said "hai hai" which could be translated as (yeah yeah).
It's not a bad thing to say, but it sounds really informal. I think the problem lays in Fushi's tired/unmotivated tone, which implies a "whatever you say, just shut up" feeling. Soooooo, the teacher got mad.
Edit: I looked it up again and Fushi did NOT say "hai hai" (I thought it was but checked later and nope).
Fushi said "ああ" which according to Google translate it translate into "~a~a". A freaking prolonged vowel without any motivation. I would get mad too tbh