It's going to take some work to get it done, but getting the ball rolling starts with people discussing ideas and getting those ideas out into the public, and I suspect plenty of parents would get on board with it, and school boards would come out against it.
Any time there is a school shooting, the local school board, the school administrative staff, the LEO administrators, and the responding officers, are all automatically under investigation, and may be brought to court on charges of accomplices to murder.
If they can't prove that they had a plan in place to prevent a school shooting, or at the very least making a concerted effort to implement such a plan, and that they executed to that plan to the best of their ability, they run the risk of being found guilty of accomplices to murder.
If anyone is found to have subverted or compromised any part of a plan to ensure the children's safety, like by propping open a door, they are also guilty of murder.
This is already being done if in the commission of a crime, there is a death that results from the crime being committed. Any time there is a shooting in self defense, there is an investigation (even if it doesn't amount to much).
It's time anyone who works in any area of government gets held to the same standard as the rest of us who pay their wages and benefits.
It's probably going to take a law to make it happen.
Home school.