If I saw some guy with a gun, my first little thoughts would probably be to run and hide or something
I'm a wuss...
"There are over 1000 people at my school if a guy came round shooting then how hard would it be to throw a chair at the fu**er."
Well when there's a bunch of crazy scared people running around trying to get out of his way, it would be kinda hard to sneak up behind him, cuz chances are he'd be looking around to make sure none of those people were coming up behind him or something to attack him during all the commotion (if there was a lot of commotion cuz maybe the hallways and stuff were fairly empty I dunnos)...
Like I said, it looks like his goal was to kill teachers, but 2 students were too... were those students people he didn't like? Or were they people who tried to stop him?