The part that didn't really work for me was the school itself. I'm sure there are buildings that are that big, and that nice, with open campuses and things, but it didn't feel like a high school to me. The kids never seem to be in class, and the building feels vacuous - they show empty classrooms, and when kids are in the hallways, the halls are near empty. It feels more like a community college or something with the amount of space and freedom (the students never seem to be in class) they have. The science classroom is a bit too staged as well, with students asking questions that aren't realistic while they were discussing electron orbitals and energy levels (let's ignore the fact that the chalkboard notes were showing a vast array of concepts that wouldn't have been necessary for what appears to be an introductory lecture.)
There's a part of me that wonders why Van Sant focuses on the students so much, showing almost no adults. I can recall a principal, a science teacher, a drunken father, and a leader of an Alliance meeting. All the other adults are faceless, and even the majority of the above mentioned are nameless.
Anyway, I liked the movie. It doesn't pretend to have answers about school shootings. It just shows a possible event.