I'm not surprised they're having school, but I don't think it means they know what happened. Maintaining a sense of normalcy is important for children who've had something like this happen in their little world. Plus, many families don't have a stay-at-home parent, and would have a really hard time finding a caretaker for their child(ren) on a weekday/daytime on short notice. And that would lead to a lot of makeshift, less than ideal arrangements -- in a town where a 7 year old just disappeared and may have been abducted by *anybody* (including possible fill-in caretakers).
Also, I'm assuming investigators will be at the school Monday (though probably in a low-key way), and we know that special counselors will be there. I expect investigators will have briefed the counselors on things to look for that might provide a clue as to Kyron's whereabouts. It will be a good thing to have all his little friends, and some of the older kids (one of whom might have done something to him) there and under observation. Some kid may have been told "don't tell" by Kyron or by somebody who took Kyron, and will start shows signs of nervously keeping a secret. Or an older kid may be trying -- and failing - to act nonchalant about the whole thing, when in fact he knows exactly what happened because he did it. Or a child may have previously experienced inappropriate contact from a school employee and never told, and just be starting to show signs of wondering if he should tell, because he's just starting to realize that there could possibly be a connection between his experience and Kyron's disappearance.
And lastly, there is only one school in the whole world that I'm 100% sure no child is going to disappear from tomorrow, and that's this elementary school. *Every* adult will watching the children like hawks, taking attendance and headcounts obsessively, etc, *and* there will surely be at least a couple of investigators on the premises all day, watching every adult closely, making sure every adult that comes anywhere near the school/children is identified, etc.