If you are applying some sense of reality, then Susan would have to have been a cop before working in the school.
It probably means she was working under cover - which would explain why she was such a useless teacher.
If it started with Leah's boyfriend then maybe he was the reason she was there - to investigate him - probably for dealing most likely, and she was trying to get evidence.
Her 'rep' may have been 'engineered' to attract him, and then she may have not been able to resist the MC.
The girls and the MC getting her fired effectively blew ( no pun intended
)her cover and she has gone back to normal duties.
The unusual thing is that she was in a police car alone - no partner?
If she is going to become a member of the harem then she will need to reveal the truth, that the boyfriend was a nasty piece of work, at some stage so Leah accepts her.
Who knows - but it's plausible.