I suggest you cut and paste your post into the behaviour forum - Tom Bennett is good at giving positive, practical advice. In my experience, the most important thing for sorting out behaviour is that the students know you mean business. This means that students who are talking over you or being rude ALL receive sanctions. Even if this turns out to be half the class. The first few weeks will be hard work following things up but gradually most of them will come on board. A few might not - but we can all deal with a few difficult students. The important thing is to crack the 'mob' mentality where a few ring leaders have the upper hand over you and the rest of the class look to them rather than you for leadership. Don't be frightened about asking for support from Heads of Year or whoever if you need it. This is why those people get paid extra money - to support people like you! For the sake of all your students, but more importantly you, it is vital that you get this issue sorted. As to when to call it a day - difficult to answer - but I would give yourself at least until Christmas, if not the end of the year because it takes time for these kinds of issues to improve.