Now, we all would like a lax teacher as our ideal – but the reality is disappointing. As the teacher could be great, but your classmates will exploit the uncontrolling teacher (and yes, I know that uncontrolling is not a word).
So, the best thing to do is try to position yourself as far away from trouble as possible in the class – and tell other teachers if something is going on. Start by trying to tell the teacher of that lesson what is going on, but because they are lax – they are unlikely to act.
It is the students that you have to deal with in this situation, and have a look at my other posts on how to do that!
How to: School 