"Elephant" by Gus Van Sant (2003)

Here is the film who shocked me! I put a link,you can watch the whole film for free and online.

Elephant is a 2003 crime-drama film written and directed by Gus Van Sant. It is set on the day of a massive school shooting. The film takes place a short time before the shooting occurs, following several characters as they live out their school lives, unaware of what is about to unfold. The film stars mostly new or non-professional actors, including John Robinson, Alex Frost and Eric Deulen.

This is the second movie in Gus Van Sant's Death Trilogy - the first is Gerry and the third Last Days; all three are based on actual events. Elephant takes place in the fictional Watt High School, in the suburbs of Portland, Oregon, and chronicles the events surrounding a fictional school shooting, based in part on the 1999 Columbine High School Massacre although the end of the film stresses the "similarities to actual events are purely coincidental" disclaimer.

The film was generally acclaimed by critics, and received the prestigious 2003 Palme d'Or at the Festival de Cannes. As the first high-profile movie to address high school shootings since Columbine, the film was controversial for its subject matter and possible influence on teenaged copy-cats. Elephant received an R rating from the MPAA.

Here's the link where you can watch it for free and online :)
http://www.watch-movies.net/movies/elephant/

Maybe you've already seen it,it's just in case if you haven't!