Does the ruling on masturbation change if a university student is compelled to it due to mixing and seeing forbidden things at the university, despite his attempts to lower his gaze?

Masturbation is forbidden and is not permissible unless refraining from it would lead to committing adultery (zina), in which case it becomes a matter of choosing the lesser of two evils. What you mentioned about the hardship due to mixed-gender education does not constitute a necessity that permits masturbation. A Muslim should avoid places of temptation and strive not to enter mixed-gender universities. If one is compelled to study in them, then he or she should minimize their presence there, lower their gaze, fast, which curbs desire, and marry if able. Furthermore, one should frequently perform prayers and remember Allah, as these protect a person from indecencies.