Well, truth be told and ignoring next chapter spoilers, Satou is doing the right thing, considering a teacher going out with a high school student is still pedophilia. Satou is being hated the same way any third wheel appearing out of nowhere to generate completely unecessary drama is hated
That's not pedophilia.
Oh dear, are we going to get into splitting hairs with "it's really ephebophilia" and the like? In any case, it's pursuing a relationship with a child or at least legally a child, as well as an abuse of power since she is a member of faculty. So the objection really doesn't hold much water because it's still bad no matter what names you apply.
All of which of course is avoided if they wait until graduation, which really isn't that far away. Then both are legal adults (although the brain is still developing into the mid-20's but nvm) and there's no professional ethics to deal with.