That Phantom teacher was particularly harsh to Sarasa. He was only a bit technical bit caring with others. Only harsh with Sarasa.
Usually in anime it means Sarasa is a talent monster. That teacher is only harsh to Sarasa because she did not show what she's capable of, she only used her mimic ability without too much effort: only some hours the night just before the assignment.
When the other 3 really worked the best they could in the very short 2 weeks they had.
I guess he wanted her to show what she's able to when she works hard on something and also show the begginings of Sarasa as an actor. At this stage he could care less about quality, he just wanted to witness work and the will to start something.
Those thoughts come from the fact that future Ai told us Sarasa has to be something big in the future.