Sandy Hook -- Lanza was a 20 year old college student. Please explain how the Sandy Hook Elementary school could have prevented this by "not hiding information about him".
Boulder (Columbine?) -- the shooting was from students who had been bullied. As a result of this shooting nationwide campaign to address and deal with bullying in schools was rolled out. I attended multiple trainings (Professional development and school meetings) every year discussing what we were doing. It was very high priority ever since the Boulder shooting.
Parkland -- Nikolas Cruz was expelled from the school the previous year. We still don't know why, but that means he must have done something very serious that the school dealt with in a very serious way (extremely difficult to get expelled these days from a public high school) and whatever he did was not criminal and was never in a public court. Again, you don't tell us what more a school is supposed to do. Since we don't know why he was expelled or anything more at this point it is baseless to assume that the "hidden information" is the key since we don't know what it is. By the way, he was 16 when he was expelled. Do 16 year olds have the right to privacy? Can they sue if you destroy their reputation? Just to put this into perspective, 2,600,000 students get expelled every year. So yes, perhaps 1 out of 10 million will come back to attack the school in some way. But you want to slander all 10 million? What exactly will that accomplish? Now perhaps 10 will be angry enough to come back to shoot up the school?