monkey see monkey do

how do you begin to stop other children from taking part in bad behavior when one child sets the tone for monkey see monkey do on a daily basis spirals out of control. spitting,laughing, back talking, running out of the classroom and etc .they are just three and four year old children.  

[Sandman2]

   At that age you really can't reason with the kids.  You have to stop the behavior before it starts or gets noticed.  Identify the one child who is the leader and work on that child.  Teach good behavior, immediately give a timeout for bad behavior.  Be very consistent.  Only pick on one or two behaviors to work on at a time.  Expect to take perhaps up to three weeks to get a behavioral change.  Parents can't help too much because the consequences have to be immediate.  However, they could work on things at home like spitting or running in the house.  Good Luck.