Plus, some of the injuries and fatalities seem to have been the result of the shooter firing THROUGH the walls, into classrooms he could not enter.
Locked doors have value, but that value is reduced if the walls are not bulletproof, or if there are windows. Making doors and walls bulletproof to 9mm handgun ammo is good. Making them resistant to 5.56 NATO (what M-16, some Ar-15, and others use) is better, making the resistant to 7.62 (AK-47 and others) is even better. At this point though, you are getting into the pretty heavy armor.
To meet the NRA's idea, we would need schools with few to no windows, at least on ground floors, unless we go with "bulletproof" glass, with is thick, expensive, and only stays "bulletproof" for the first few shots. Walls would need to be resistant to 7.62 ammo, which has been used by a number of mass shooters, including the Las Vegas one.
Prisons are like that.
Beside adopting those slight tweaks to school construction one would suggest making a few small adjustments changes to the school uniform? Perhaps something like this?