I'm going to take a stab in the dark here for you, EII.
We've all been in high school, we know what happens there, so I'm going into this little spiel assuming that you all know what I'm talking about. Seeing as how he only shot at one particular group of kids, these were people who probably socially terrorized him. They probably gossiped, spread lies, and slandered his name. He may very well have been attacked physically in locations where he should be safe (the bus, in front of the school, etc.). Apparently these kids were kind of popular, so the rest of the school follows suit (the groupthink mentalities I saw in high school are oddly disturbing). He probably felt isolated and threatened. No other options in his delusional mind, some base instinct kicks in and he turns into a killer. He strikes at the top of the pyramid (popular kids) of his victimization (head of the snake and all that). Suddenly we have a Drowning Pool song on our hands. His mission accomplished, he flees, and turns himself in.
Again, that's just a stab in the dark, but it seems to be a pretty common storyline for school shootings.
It's a good thing he went to a nice high school. At my alma mater, people might have started shooting back.