Good but not original
First of all. This is way too similiar to the film Elephant.
You totally ripped off the scene where they tell the friend outside to go away, aswell as the teacher scene where he says "bitch". You also took the soundtrack too, with the piano tracks. You did however choose good music (<3 Godspeed you black emperor). All that negativity aside, the graphics although far from perfect were fairly detailed and most of the animation was well done. I like how you kept the original Columbine quote of "do you believe in God?" but you should have shown them commiting suicide at the end for people who were too lazy to read the letter and dont know the actual Columbine story. The music was good, the voice acting was a little... cheesy. It would of been cooler, if you kept the whole thing silent until the end when they ask if they believe in God, you'd have to lose the bit where they let their friends get away, but that never happened in Columbine.
Anyway, good work, the effort put into this is clearly visible, just try not to take too many ideas from existing media.