There are a few things you can't fix. You can't fix crazy people, and you can't fix stupid people; the rest you can work with.
The shooting at any school in America is impossible to stop when you have a suicidal individual bent on death. But with two things, you can minimize the damage:
1) Solid doors with one or more heavy chains out of reach. The gunman would have to shoot the door off the hinges, and that takes time and bullets.
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2) Place a can of hornet spray in each classroom, or bear spray. These will shoot a spray 20 to 30 feet. In the case of the hornet spray, the attacker will not proceed very far after being sprayed by one or more teachers or school staff. Retail price is $5 to $6 per can, so for $200 or so you can – at the very least – slow down the person bent on destroying lives and property.
These things won't always stop the tragedy but will certainly minimize the worst.
Think outside the box.