[rhcom2]

I don't think it is about actually hiding or being secret. It's just about stalling. Getting through a locked door takes time that responding officers can use to get to the school. Most school shootings stop as soon as the first responding officer engages with the shooter.

[AstralStorm]

Locked door will not stall a person with a gun willing to shoot through said door. And as they know about the drill they will not be fooled into thinking nobody is there. Plus they can just shoot the lock off. And building a real barricade takes a long time, strength plus skill.

It will prevent any solid response from you though, other than jumping out the window (if possible).

This is why people are trained to evacuate not stay putt during danger. Or stall it actively if they can. Same principle as in fires, earthquakes, floods, bombings or other terror.