Your a idiot.The door was not propped open. The police lied but the Texas DPS corrected the record. The problem is that the door didn't automatically lock - something easily fixed for about $100 or so.
You're probably right, initially, about children opening the door when someone knocks. That's easily fixed by explaining to children that they don't open the door, even for people they know or even teachers. They'll understand and comply. Tell the teachers and staff that knocking on the door to get someone to open it for you is a firing offense.
I do live in a dream world: I dream of seeing children protected in ways that will work and are meaningful rather than simply banning one tool which would simply be replaced by the next tool, more deaths, and the next ban, followed by the next tool and more deaths and the next ban.
Just how many children are you willing to watch die in order to get your desired gun ban?