Haruki went from sort of a villain to sort of a hero in one chapter. Mochida went straight from neutral to villain. If one girl following a teacher around was enough to start a rumor that lasts years, imagine the insane amount of damage a credible rumor about her sexuality could do.
I wonder if Haruki will warn Sei or Aya about this. Not selling them out is one thing. Actively trying to stop Mochida's plan is another thing entirely.
This is Sei's fault mostly. She seems to be a bit of an exhibitionist. Well, maybe that isn't the right term, but it is the closest one I can think of. Basically, she likes flaunting her relationship status with Aya to others. She knew Mochida was listening to their conversation back in Chapter 3 and continued anyway. She sent that picture to Haruki as well. Maybe she had some noble intention with Haruki, like trying to make her realize her love for Aya. Maybe. I think she just gets off on it though, quite literally.