Wouldn't it depend on what the body was incased in....like a closed container that was hidden inside a wall?
Warning: gross discussion ahead.
I doubt that scenario because of the nature of walls and the nature of bodies.
Most walls do not have vast open spaces inside of them. There is a space between the walls of less than four inches, maybe six inches in luxury buildings where sound dampening is a concern. The spacing between the studs inside the wall is usually somewhere between 16 and 24 inches.
So placing something inside that cavity means that the something has to be no more than four inches by sixteen to twenty four inches.
It is just barely possible to stuff a small body into a wall space (see Annie Le). Kyron surely falls well within the size that could be placed inside a wall. That relies on the body being flexible and somewhat compressible.
A container would not only add bulk, it would be rigid and not compressible.
Now, it is just barely possible that his body was swaddled in many layers of plastic sheeting which were extensively taped up.
However, decomposition involves the generation of gasses, which tend to lead to swelling, pretty dramatic swelling.
If Kyron's body were wrapped in plastic, the swelling of his corpse would inevitably loosen the plastic wrapping. The gasses would make their way outside of the plastic wrapping. Fairly rapidly, part or all of the school would be permeated with that musty "something died here" smell.
There are exceptions, of course. If his body were placed where the ventilation system carried all air upwards and out through the roof, the school itself might not smell. However, dogs deployed on the roof (and probably humans as well) would be able to smell the scent of decomposition.
I seem to recall that LE did specify they have checked the whole school, including the roof.
There is a tiny, tiny possibility that Kyron's body is still inside the school but I consider it highly improbable.