They should be closed and locked regardless. That should be normal protocol - those needing in knock on the door to be let in (should be a small window in the door to be able to see out). That should be basic security protocol.
One would think the sound of gunfire would have alerted everyone to lock the doors instantly.
During training for this scenario several years ago, the local sheriff’s office said their policy after Sandy Hook was to go in immediately—not wait for SWAT team backup.
Very troubled by some of the information coming out about the very long delay in getting in there. Even if they weren’t killed by initial shots some of them may have died due to lack of medical attention that could have saved their lives. Something is really messed up if those reports end up being true.