What is the optimal course of action with a Muslim sister who thinks ill of me despite my good intentions, and should I continue to endure it or seek other options?

Mixing in schools and universities is forbidden. And whoever is afflicted with it and finds no alternative, then he must fear Allah, lower his gaze, seclude himself from women, and not speak to them except out of dire necessity and to the extent of that necessity. He must abandon behaviors that arouse suspicion, avoid girls, and if it is possible to change the group in which the girl is present, then he should do so, otherwise, he should limit interaction to the absolute minimum.