That teacher had a great character arc. He was initially being introduced by others as someone who primarily cares about noble heritage, which was already subverted when he was shown and acknowledged fmc's exceptional abilities right away. Then it was revealed that he thought that exactly due to her being a genius she'd be bad at teaching others (which is often the case and makes sense) to accept her in this regard as well now.
In two dozen other stories he would have been a one-dimensional villain, because lowly commoner bad, but here he really just cared about the students from beginning to end.