Seeing as we don't really have an example of Itsuko as a teacher, it's hard to say exactly how she'll do. But regardless of both arguments, I think it's best to keep in mind that it is a [i]school[/i] for assassins and if she can't do her job properly by teaching [i]all[/i] of the students she's assigned, not just the one's she's in a flirtatious relationship with, then she likely won't be a teacher for long.