What galls me is apparently teachers were ordering students to turn off their cell phones. This is massively unwise: parents get irrational when their children are in danger, and when they don't know anything at all, the panic makes them fucking psycho. We've seen this in every previous incident.
Both schools and families need to have a plan for disasters that takes good communications into account. Teachers locking down classrooms shouldn't order phones off. Anybody who wants to get a message out should be allowed to, preferably using text only. Text messages are sent through overhead channels, consuming very little resources. Phones will retry sending at intervals if the cell is congested, as it would be in any kind of crisis. If the kids have to go voice, the calls should be as brief as possible--30 seconds or less. They need to pass along exactly what room they're in, that they're okay, and the folks should chill out until they're released.
Parents ought to keep an envelope someplace close at hand containing a copy of their kids' schedules, a building map, and a list of contact phone numbers for their friends and their friends' families. Why the phone tree? "Hi mom, it's Andy, the school's locked down, I'm okay, I'm in room A-214, Bob is with me, he doesn't have a phone, would you call his folks and tell them where he is and he's okay too?" The schedules and building map give the parents an idea of where to wait for their kids to be released. It has to be pretty scary to see kids streaming out of a building, and to not find yours. Chances are, they're fine, they're just coming out a different door. There should be a backup meeting place agreed on in advance in case all else fails, preferably someplace with a pay phone that can be used to contact each other, through a third party if neccessary. (Just like if you were at an amusement park.)
The shooting scenario is the worst nightmare, but this kind of plan is useful for other kinds of disasters--gas leaks, freak blizzards, floods, fires, whatever.