I feel that 'The teacher is sad to be leaving the school' is a better way of expressing this in English. But would that be put differently in French? I'm not sure, so haven't suggested it.
I wrote the same thing. I will report it.
Again , you're marking us wrong for a perfectly correct interpretation , 'from the school " , to me this just interprets directly and I really can't guess what the meaning was meant to be.
The meaning was supposed to be "to be leaving the school"!
Your version implies that he was just going directly on to somewhere from the school.
You give the impression that you have misunderstood the context and Duo is quite right to reject it.