BBM - I don't think they knew that Kyron hadn't been in school all day -- after all, that's where SM left him... I also don't think they heard that from the bus driver, I BELIEVE they got that information from calling the school (if I remember reading correctly).
I have put my own self in this situation a few times since Kyron has gone missing and as random information has been brought forward -- if I waited outside at the bus stop for my child to get off of, and he never did, I would have called the school right away and asked, "Is by chance he still lingering around? He didn't get off the bus! Did he miss it?" and if they told me .. "Well, your child wasn't here all day!" My heart would literally stop... I'd jump in a car and high tail it to the school -- WHERE I HAD LEFT HIM. I don't think I'd call 911 (fearing the worst), I'd get there as fast as I could, in hopes I'd find him there, standing outside the school, maybe wondering "Where'd all the buses go??". Weird reaction? Maybe... but jmo.. again, of what I'd do.. so I can honestly relate to their reactions.