I'm gonna go out on a limb and guess that the "day before graduation" was them locking her in a school bathroom where she got assaulted if not outright raped by some creep. Feels like the sort of escalation that would fit in at the tail-end of everything else they've done to her, and also would make sense with the tone of the series, given the author and their other work.
That said...Kitami is starting his own villain path now, potentially. Which makes me wonder if he'll pull back and get it together with regards to Himari, or if he'll do something that puts him on her "list", as well. Or, he ends up killing her himself at the end, out of jealousy for "everything he did for her, not being reciprocated (in the manner he wants)". I could see this going a few different ways once Himari's revenge is complete, but the pieces being laid out here and now have me wondering.