You def have a point. Though my issue with this is that it's too unbelievable. Like the OP said, they've only been there a handful of times. Ok I am going deeper than I really should, but unless they submitted a portfolio or something, this teacher doesn't know him and his base of work. You can have a good eye for art. You can see that their art is missing something, but how she reacted was weird for someone new to these students. I think it'd be more believable if they had been working with her for a while then the teacher saw the change in work. I think what she said was spot on but too dramatic for this brand new character.
Perhaps, or perhaps 3 times were enough to see that his work is for some reason lackluster.
Could she have used more tact in the way she phrased it? obviously.
Now there are a few reasons why a character is portrayed like that.
It could be that they have been going there for more than 3 times, but the others were "off-screen".
It could be because the mangaka wants to "force" the "confrontation" we witness(between inuyama and toyoda) to happen without having to draw 20 extra panels for that, and well, let's face it, kind and encouraging words from the teacher would not make that happen, a "slap in the face" statement like the one she said would.
Then there is also the change that the teacher is acting the part of the "asshole" in order to try and push the students to push themselves and show results. Or at least help someone who seems to not be able to cut it to save wasting money on lessons that don't seem to help them.
But yes, there is the slight change that the mangaka wanted to portray her as a bitch...be he hasn't done that so far, at least not to that extent. So I kinda doubt it that this is the case.