As a teacher and professor of nearly every grade level I can attest to the fact that it is true that positive reinforcement is way more effective than punishment. By positive reinforcement I am refering to rewarding good actions rather than punishing bad actions.
One of the best reward programs that I have is very simple. Each time I see a student doing the right thing, I put a piece of candy, or a cookie, inside a little bucket with the student number written on the outside.
This works for EVERY grade level, since everyone loves candy. Just get a small container, assign a number to each student and say that, everytime they do the right thing, place candy, gum, or whatever, in the bucket. Likewise, take it away if they misbenave.
However, there is something else. In difficult groups such as middle school and early high school students, the best solution is prevention. Prevent hot situations by keeping the students busy and engaged. Once they are engaged there is very little they will be bothering each other about.
So, plan ahead and plan accordingly. Pretend that you are one of the students: What would your teacher have to do to keep you busy and satisfied in class? Then, do the same.