We could stop it. But we won't.

so, leave

Short of taking every gun away from everyone, what do you say would have stopped this?

One entrance/exit with security would stop it
Locked classroom doors would stop it
Insane asylums would keep the lunatics locked up
Stop coddling kids...this NEVER happened a few decades ago. Kids weren't coddled back then.
Allow the cops do their jobs...stop micro-managing them while they are doing their jobs

Not every gun.

(1) Ban the sales of military-grade assault weapons, and (2) raise the purchasing age for all guns to 21 (for most private citizens — I’m fine with exceptions for military or other lines of work who need guns under-21, since they’ll receive vigorous on-the-job training).

The Uvalde shooter had just turned 18 and used assault weapons purchased legally. So, either or both of these measures would have stopped him.

One-entrance, one-exit school compounds isn’t feasible, IMO, since that would present hazards in other areas such as fire escape. What you actually need is as many exit points as possible, to allow children and teachers to flee. My own high school had an exit door leading out of most classrooms.

what's an assault weapon?
The rest of your post is nonsense. They have locks that allows for exits. All schools have them. Use them accordingly. Place those kind of locks on each classroom door.