Assalamu alaykum, Shaykhs. I am a medical student and have a vast syllabus. There are examinations frequently. Some teachers' classes are not good and not beneficial. So I read the exam topics in those classes, otherwise I have trouble to cover all topics and will have trouble in doing other works in my home. I usually sit in the middle row so that the teacher does not notice. if I do not do this, I will not have enough time to read the textbooks properly, which I personally do to increase my detailed understanding of all subjects of my whole undergraduate course. I listen carefully to the lectures of those teachers who can deliver lectures well. I do not give any proxy, attend the classes, and sit far from those teachers who cannot give lectures well to read exam topics. Is this a sin?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

If you do not benefit from the lectures of some teachers given their poor explanation skills, as you stated, then there is no harm in studying other subjects or revising for the exams or the like during such lectures.

Allah knows best.