No child is perfect, but teachers and parents know that there are some behaviors that are unacceptable. They are the ones that are likely to harm children physically or emotionally. Two year olds can have terrible temper tantrums and sometimes bite other children.
Threes can hit hard, push, and even scratch.
Fours can hit even harder and send a former friend into tears with a choice name you are sure no one in your family ever used before. Fives can look like they are playing happily when closer examination reveals a group that has been organized by a child who gets loyalty by bullying. The earliest and most troublesome unacceptable behavior in the preschool is biting. It can sweep through a preschool like the measles. Preschool teachers groan inwardly.