The moment a child walks into a new classroom, teachers and classmates form an impression based on minimal information. Yet, little is known about the accuracy of such impressions when it concerns children's social functioning at school. The current study examined the accuracy of children's, teachers' and adults' impressions of 18 unacquainted children based on thin slices of behavior. The likeability, popularity, prosocial behavior, aggression, and exclusion of these children were judged by 101 children, 79 elementary school teachers, and 68 young adults based on 20-second video clips. Judges were better than chance in predicting popularity and prosocial behavior, but worse than chance in predicting aggression and exclusion. Female judges were more accurate judging social exclusion of same-sex than other-sex targets. Teachers were more accurate than children in their judgments of prosocial behavior. The current study shows that confidence in one's impression of aggression and exclusion in unacquainted children based on minimal information is not warranted.