Your son comes home from class with a bite mark on his arm from another child.
What you're tempted to do: Confront the child's parents, and shoot off an angry e-mail to the teacher.
What you should do: Preschools exist in part to help children learn which social behaviors are acceptable and which aren't. Hitting and biting are common at this age. Although that doesn't make them acceptable, your son's teacher is the right person to handle the problem. Feel free to set up a conference where you can express your concern: "Would you please keep an eye out when our son plays with Max, so this doesn't happen again?" Also, make sure the teacher has spoken to the boy's parents. There's no reason for you to contact them directly, though, unless you're friendly with them and feel that not mentioning the incident would be awkward.