I may not be correct, but I think it was because the teacher was working under the Duke's wing and he was a researcher, I think he found out about something and he became a target of the emperor's faction? That was why in a chapter before the Duke went to his old academy and went to the teacher's office which became a mess because the emperor's faction wanted to find the teacher's research? I dont know it's just something I thought of when he mentioned the teacher, Ardea.

Who's Ardea? I don't remember her saving anyone