What good would keys do?
I don't know the details at the school, but I think large glass windows in doors are pretty standard in schools, as well as windows in classroom walls in some. Principals like to walk the halls and see what is going on in the rooms. (These days it is also due to fears that teachers might more easily molest kids if they can't be seen from the hall.)
With a glass windowed door, all the perp has to do is break the window, turn the inside knob, and walk in.
I don't think keyed deadbolts in a classroom would go over well with the fire marshall.
Maybe an emergency device that could be secured would be feasible (similar to the old 2x4 in brackets setup but harder to defeat. Also, school doors must open outward). Again, there are very strict fire laws about school doors due to some horrible tragedies years ago.
Can’t open the door by turning the knob after it has been locked by key.
But you make a point. If there is no where to hide and there is not bullet proof glass you could just stick your gun through a shot out door window and kill them.
For that matter, you could be outside, pop up, and kill them all through an outside window.
But the reason a lawsuit will prevail if allowed to continue is because they did not follow adequate security procedures by giving the teacher a key so that she could lock the door . That was the Standard Operating Procedure for a lock down and they didn’t give her what she needed to lock it down.