Hm, i am not 100% sure and you didnt mention it. But noriko brought him to her sister after all that. I mean wasnt she doing a commercial or something? Oh, i hate it when i cant remember something clearly.
After being reunited in 'What Could Have Been', talking in his office in 'What Is', Noriko then brought Sensei to Niki in 'What Was'. All 3, Main Events, in Chapter 2 that occur the day after the last.
The thing I find odd about this is it implies there was some unknown reason for him to stop tutoring instead of it being because he became a teacher, which would be easy for Ami and Noriko to assume so there must have been a gap. So something made him stop tutoring but didn't stop him from becoming a full blown teacher? Right now I'm thinking he had a similar issue with self control that he's having now. He stopped tutoring because he didn't trust himself around wizards and later on thought he would be better off with teenagers.
Edit: He may have quit tutoring specifically to continue teaching Maya come to think of it.
It seems like Maya did something, tbh.
Maya apparently made a deal with Noriko, prior, that was intended to prevent her from leaving the Old District to come find him:
And Maya Prime apparently tried to hide this deal when Sensei asked:
What she did though, isn't clear, but she definitely knows what she
could do: