What is the ruling on staging a sit-in and demanding the university president's removal in light of security forces storming the university campus and assaulting students? And is this considered a rebellion against the ruler?

The reason for police intervention cannot be determined, nor whether they are oppressive or not. You should ask the scholars in your country about this. As for demanding the removal of an institution's director for a legitimate reason, it is permissible and not considered an act of rebellion against the ruler, provided it adheres to legitimate Islamic guidelines. Umar ibn al-Khattab dismissed Sa'd ibn Abi Waqqas in response to a complaint from the people of Kufa, despite his trust in Sa'd, and this is evidenced by the hadith of Jabir ibn Samura.