Is it permissible to be absent from optional or mandatory classes at the university and invest that time in work, knowing that the university is mixed and that there are books that suffice for the optional classes?

If a student can attend and benefit from their teachers, they should do so, and they ought not to be absent except out of necessity. If they are absent, they should be keen to make up for what they missed. They are not sinful for absence without need, as long as it does not lead to cheating or falsification. However, if the study environment is mixed (gender), then it is not permissible for them to join it except out of necessity or dire need.