Use books, puppets and role play to teach children appropriate behavior. Young students need to learn about the rules they should be following. Teach this with resources that illustrate what's "good" and what's "bad." A book like "No David!" is funny, but also helps you to explain your expectations about behavior.
Establish standard punishments for breaking rules. Students should understand what will happen when they don't follow the rules. You may want to put them in the corner or in a timeout. You could also take away privileges or other rewards. Whatever you think is most appropriate for your situation, be consistent. Use the same punishments for all students every time someone breaks a rule.
Give students a warning when their behavior starts to cross the line. Because young students don't always know right from wrong, it helps to remind a rule-breaker that what he is doing is not appropriate. This gives him the opportunity to adjust his behavior before you dole out the punishment.
Be sure to keep parents in the loop about their children's behavior. Parents may be able to let you know about issues at home that are causing behavioral problems in the preschool class. However, you should also be sure to let parents know when their children behave well.
Offer rewards to students that behave well. Positive reinforcement often works better than criticizing bad behavior. Before class starts, establish a reward system--such as giving out a sticker every day to students that are "good"--to encourage good behavior. Sometimes the best punishment is simply not getting the reward that other students received.