The producers of the movie wanted to cast talented amateurs from outside Hollywood in the main roles, and that meant a nationwide call. But their budget was virtually non-existent, so they did the next best thing and started stalking real schoolkids. One of the film's talent scouts went to Westchester, New York and walked around a middle school spying on kids in hallways and classrooms because, Jesus, the '70s really were a different time, weren't they? On the last day of "scouting," Stalker Ace found a young Matt Dillon skipping class, wandering the hallways acting tough, as teenagers often hilariously attempt to do, and brought him to meet the director. Presumably in the back of a windowless white Econoline van.
20th Century Fox
Which would explain his choice of mustache for There's Something About Mary.