Disturbing Behavior (1998) starring James Marsden, Katie Holmes, Nick Stahl, Steve Railsback, Bruce Greenwood, William Sadler, Chad Donella, Ethan Embry, Katharine Isabelle (of "Ginger Snaps" trilogy)
IMDb synopsis: After the suicide their son, the Clarks decide to move from Chicago to quiet Cradle Bay Island, seeking a peaceful life for siblings Steve and Lindsay. When Steve starts high school, outcast Gavin Strick befriends him and introduces him to also-rejected Rachel Wagner. Gavin tells Steve about the cliques and defends his theory that a sinister force brought about the behavior of the annoyingly perfect "Blue Ribbons", a group of good students who wear identical jackets and gather at the Yogurt Shoppe. Further, he tells that he had witnessed Blue Ribbon Andy Efkin killing their schoolmate Mary Jo, who is indeed missing, and that a policeman covered up the crime. Steve does not believe Gavin, but, when his friend is submitted to the treatment of Dr. Edgar Caldicott and immediately changes his behavior, joining the Blue Ribbons, Steve and Rachel decide to investigate the mystery and discover the dark secret of this perfect behavior.
Here's Part 1 on YouTube: http://www.youtube.com/watch?v=knYzOw4jReY