I agree with Chuy: "Don't chase each other in the classroom." That tip he gave to use as a direct command could really be useful! Danny Aguilar also has a good way to express the same idea, although in a more general sense with the expression "no horseplay." This would apply to more than just chasing each other around the classroom and would include any sort of rowdy or rough behavior in general.

Thank you, Chuy and Danny!

the best phrase to get the point through is "no'horseplaying."

Best way would be: Don't chase each other in the class room. As direct order or you can use: Stop running in the classroom.