Preventing Another School Massacre

Phil Proctor has a simple idea I haven't heard anywhere else:

Why couldn't the victims of the most recent massacre have simply LOCKED THE DOORS of their classrooms against the murderous assault?  We've heard heartbreaking stories of attempts to blockade doors with desks, chairs and bodies, resulting in heroic sacrifices, but it appears that none of the rooms were equipped with inexpensive inner bolt-locks and peepholes to insure that the students would be safe during their period of instruction. That approach seems to be working to keep our airlines safer (although we've also armed our pilots) so why not apply the same principle here?