Here's a little known fact.
The police had the Michaels near the school staked out on Monday morning.
Now some conjecture.
The sniper saw the police when he drove to the Michaels, got really mad and drove off in a rage to seek another target. The school was a last minute choice, a target of opportunity, so to speak.
The sniper changed his MO at the school. He got out of the car and shot from the ground. He left a shell casing behind.
If what you say is true, I trust all the Michaels stores in the area are going to be under protection for a while.
Very possible.