What's your plan?

Are you going to take the kids into the hallway and possibly run into the shooter?

Like I said if the walls, doors and locks are substantial enough to keep the shooter out, its the best plan there is other than taking out the shooter.   

Taking out the shooter won't get done until law enforcement arrives unless you have teachers/administration/security packing heat, which I'm not opposed to and in fact I would support having staff that was trained and evaluated (mentally) for concealed carry.