Explain what Mr M means when he says, 'My apologies for ... high and low' (lines 1-2).

Here are the answers to your questions:

4.2.1
The setting of this extract is the school where Mr M teaches, likely on the school grounds or in his office.

4.2.2
When Mr M says, "My apologies for descending on you out of the blue like this, but I've been looking for you high and low," he means that he is apologizing for appearing unexpectedly or suddenly to Isabel, and for having searched extensively for her before finding her.

4.2.3
Mr M was looking for Isabel because he, and the students, missed her and wanted her to return to the school. Her previous visit was very impactful, and students were constantly asking when she and her team would be coming back.