When I subbed in elementary levels, one "trick" seemed to work well. Talk with the kids in the morning and find out what kind of reward they would like the most (going outside, having a special book read to them, a game, short movie, ...). Write a word on the board (ex. OUTSIDE) and everytime the class gets rowdy, erase 1 letter. When the word is gone, there will be no outside time. Stick to it - they will learn quickly that you mean it .