There's no worry about lost keys, because there aren't any keys. It's just a simple deadbolt lock with a thumbturn on the inside, and nothing on the exterior of the door. Also, classroom doors should either be windowless, or at most have a small window (think 6" by 16") with a blind that can be pulled down.
Just a worry about fire codes, exits, and burning to death. Every Classroom should have two doors. Locking doors are good, though you still have issues to deal with-
Students late to class?
"Surprise" guests?
The student sent to the principal's office?
The armored door that locks from the inside means LEO's clearing the room of potential hostages have to blow the door off the hinges if they can't convince a terrified kid inside they're safe enough to open the door, further traumatizing the kids. And God forbid there's a kid hiding next to the door in the blast area.
We want locks on doors, don't get me wrong. But we want at least a couple classes of people with the ability to open those doors on demand. School staff, LEO's, Fire, and First-responders. Having a teacher die in front of her Kindergarteners because they can't reach the lock and she's having a heart attack would be traumatic as well.
I don't know enough about physical security to make an informed decsion between electronic locks with a UPS, key cards until a lockdown, then keycards + Pin code, Pin Code only, or sticking with a physical key/lock combo, possibly at least somewhat proprietary like fire department elevator keys. But I do know we can't focus on the extremely rare school shooting, and ignore other school emergencies- health scares, natural disasters, fires, and so on.