A first inclination would be to panic, and then, having the bus driver immediately reacting and contacting the school will alleviate that for a few minutes, until the news comes back from the school end to say "We never had him marked as present here today" whereby the panic sets in again and thrusts out any complacency and ease with a situation which COULD have had an innocent answer (he missed the bus; he got a ride from another parent, etc etc....even if either is out of character for the child). Once the secondary panic sets in, it sounds like the Horman's were at the school as quickly as they possibly could get there. The road is twisty and turny all the way to the school and I cannot imagine you can go full bore around those curves. Once arrived, the next stage of alarms were sent out.
Meanwhile, the school has done its check of roll call, talked to the teacher whose room Kyron would be in, and have established something they can tell LE, who are now also alerted and making their way to the school, as well as to the home, where LE has requested one of the parents be when they get there, or Kyron is miraculously appeared.
It just isn't always that someone failed to do something in the beginning; sometimes it is that their hope and prayers are that the child is misbehaving in a manner that will be worth a talking to, and not a 12-day and counting search. KWIM?