A really stupid trick that I found out this year, which works, is the Countdown Technique (I feel so like Supernanny) You feel like a real Richard doing it, but for some reason it works. You stand at hte front of the room and stick your hand in the air and say: "Quiet in 5...4..3...2..1.." (putting your fingers down) and everyone shuts up. They think they are waiting for something.
This worked for my year 12 class that was all mucking aobut. I can't guarantee they will stay quiet, but if they are quiet for that moment, then you can start to make that moment grow little by little.
It is so hard when everyone is trying to make it hard and horrible for you. Hope it goes better for you soon