Brick (2006)

When student Brendan Frye's ex-girlfriend vanishes, he becomes consumed with finding his troubled inamorata. To find her, Brendan enlists the aid of his only true peer, The Brain, while keeping the assistant vice principal only occasionally informed of what quickly becomes a dangerous investigation. Brendan's single-minded unearthing of students' secrets thrusts him headlong into the colliding social orbits of a group of unique pupils.

Official website: http://www.brickmovie.com.au