I think if my kid didn't get off the bus as expected, my first order of business would be to dial 911.
As far as I'm concerned, that qualifies as officially missing.
I think the school's response to a bunch of cops showing up would have been a bit different....and (in my area at least), patrol cars and detectives can be in more places at once and more quickly than I can be, too.
I also think that there's no excuse for not having radio communications with all the buses. Once you've figured out a child isn't at the get-on point or the get-off point, it's a good way to figure out where they might be in between. (I almost think that only some buses have radios in my district--those that the disctrict owns. The hired buses may not....)
A few years ago, I answered the doorbell early in the morning to be asked by a detective if I had seen one of the neighbor's kids at the bus stop near my house. Apparently, he had been sent out to the bus stop a little early, and when his mother drove by the stop a short while later, she saw the rest of the kids waiting for the bus, but not her son. She, rightly so, panicked.
He turned up safe and sound at school, having been dropped off by another parent, who thought he and her daughter had missed the bus.
I also suspect that police involvement would help ensure that the school wouldn't be so blase' about who gets on what bus.
Scary.
Brett