So, what if the WHOLE TOWN was given some kind of drug? And some people either did not survive or had such an adverse reaction that it would be OBVIOUS they'd been heavily drugged against their will? The children may have died from the drug. They might have been dead or dying before they were shot.

Their teachers, unfortunately, may have been collateral damage. Especially Ms. Rousseau of the shot up car. It was her first day.

This would also explain some of the not so upset acting parents. They were still under the effects of the drug at that time.

The kids who survived may have just been told they don't remember the shooting. So, they remember and described things that happened right after... noises probably made by the police trying to get everyone out of the school.