I already find her to be completely despicable. My guess is that she serves as a trigger to jumpstart Ranko´s career so that Tatara can end up with Yuriko. Alright then, but why do these kinds of characters always have to behave in a borderline-criminal way?
My guess is the teacher makes Shiori go through all those painful experiences so she'll thoroughly despise bullies and then reveal what her father did, so she would rat him out. Just my two cents.