Answer: at high school
Arthur Pendragon, or Artie as he prefers to be called, is engaged in a jousting match, which he subsequently loses, when they track him down. When they first walk through the school's gates, Shrek comments that his hands are sweaty and his stomach hurts, so it must be high school. The Medieval Cheerleaders are amusing, as is the girl who asks Shrek to be her girlfriend's date.
Arthur is voiced by Justin Timberlake, who was dating Cameron Diaz at the time the movie was made. Ironically, they broke up before the movie's release.