Is righteous companionship obligatory, and what should one do if they cannot find it? And how should a Muslim student act in a mixed university with unveiled girls and immodestly dressed female instructors?

The Sharia emphasizes the importance of good companionship, for a person is on the religion of his friend. Therefore, one should choose a friend who helps him in good deeds and reminds him of Allah, for "Close friends on that Day will be enemies to each other, except for the righteous." As for mixing in educational institutions, it is forbidden and a gateway to corruption, and the general rule is to avoid it. However, if there is an urgent necessity or a dire need for it, then there is no harm, provided there is a strong presumption of safety from fitna (temptation/strife) and adherence to Sharia guidelines, such as avoiding sitting with immodestly dressed women and limiting interaction to what is necessary. A Muslim's refraining from speaking to immodestly dressed women out of fear of fitna is not bad manners; rather, it is the essence of good manners. For whoever pleases people by displeasing Allah, Allah will entrust him to them.