Learning the whole course ahead of time sounds easier said than done. But I definitely recommend pre-learning the next chapter in the course instead of relying on the teacher's explanation. Personally, I could never understand a relatively complicated math concept just by listening to the teacher. I usually need to think about it, draw things, read several different explanations, etc, to really get it. But when I was already familiar with the topic, then I could benefit from another repetition and ask questions if there were some complicated aspects.