Is sitting with lecturers who present reprehensible ideas (such as discussing alcohol and music), and not denouncing them for fear of being marked absent, considered participation in sin?

You must denounce the objectionable matters in the lesson to the best of your ability. If denunciation does not help, then you must leave the lesson. However, if your absence harms your studies, and you need them, then denouncing it with your heart is sufficient, because "what is forbidden to prevent a means (sadd al-dhari'ah) is permitted for an overriding interest (maslahah rajihah)."