Well that brings up a question I had early on. I'm not sure where this would fit, so I'll post it here.
I was a bit surprised that LE would hold interviews at the school on Sunday, and allow school to start back on Monday morning (and allow the 'field day' on Thursday as announced by the principal). It's bad enough all day passed before they had a chance to thoroughly process the school for evidence, does anyone believe they could have done so in one day (Saturday)? I wonder about that...
I think it's the same problem as with KH's truck: what could they find that would advance the investigation? I believe that is why that truck has not been impounded (no evidentiary value).
Since it is certain that Kyron disappeared before 10 am but wasn't discovered to be missing until 3:30 pm, I'm sure that all the floors had been mopped, all the bathrooms cleaned, commonly handled surfaces like door knobs, etc, were probably all wiped off. Probably more than once.
Kids are innately grubby, sticky beasts. In elementary schools, the best way is to keep abreast as much as possible, which means cleaning throughout the day. As soon as some area gets a chance to get dirty, it gets cleaned whether it actually looks like it needs it or not.
Plus there's the constant concern about trying to slow down kids passing germs to each other. That is most effectively done by wiping down commonly handled surfaces (door knobs, table tops, etc) as often as possible.
I think the public areas of the school were probably written off for evidence pretty quickly. In a building that something like 600 people had passed through the day of Kyron's disappearance, trying to track down every single fingerprint would be a nightmare and probably economically impossible.
That would leave checking non-public areas for anything suspicious, which could be done pretty quickly (it's easy to check for the presence of blood, for instance).
I think their main concern would be to make sure that Kyron was not still within the building. With the help of cadaver dogs, that could be done in a matter of hours.