From what I heard, HuskerXDM is correct.
On the news tonight they said he came in the regular door, signed in with the security officer and asked to see the asst principal.
When he was escorted into her office he shot her 3 times, then met the principal in the doorway on his way out of the office and shot him. Upon exiting the office he took a shot at a janitor and missed. As he headed out the school's exit he covered the guard with the pistol.
Perhaps if the front door guard had been armed he might have shelved the idea, then again he might not have.
Honestly I'm not seeing a lot of flaws in the plan here. He stole the gun while his father was out of the room, broke into the garage to steal the car, and followed the school's rules upon entry.
I think the kid just couldn't deal with some emotional turmoil and decided to take someone with him on his way out.
A sad, sad way for two lives to end, and countless others to be traumatized.