My thoughts are...
TH & Kyron walked through some of the exhibits and possibly completed their tour in the Library.
Saying their goodbyes, Kyron goes West from the Library, through the door into 105A, turns North through the door into the Hallway, West through the door into 106/Hallway and up the West Stairs.
TH goes East from the Library through the double doors into the Hallway, turns South and goes up the South Stairs.
TH sees Kyron as she looks West and he is at the top of the stairs. TH continues East toward the East Exit.
Scenario 1 ~ Kyron sees his friend outside of the classroom, at approximately the same time that TH last sees Kyron, and says he is going to see the electric project, which could possibly be North of his classroom, down the hallway toward 215 & 217. Something happens at that point.
Maybe Kyron is taken into the small room at the end of the North Hallway, until everyone is in class, then taken out the door on that hallway to the back playground or back down that hallway to the West Stairs and out to where a vehicle was waiting.
OR
Scenario 2 ~ Assuming that Kyron actually made it into the classroom and got assembled into their little groups with Kyron and T in separate groups, the sub/volunteer gives her group a brief synopsis of projects that they are going to look at. As the groups are getting ready to leave the classroom, Kyron tells his friend in passing that his group is going to see the electric project. Their groups part ways to go see different exhibits and Kyron gets separated from his group, maybe by going to the bathroom or to get a drink of water. (I thought read somewhere about a photo that Kyron was missing from of his class and Kyron not being in it. It had a caption that led me to believe that Kyron has a tendency to wander a bit, like to go get water or use the restroom, without telling anyone.) It appears that the main restrooms are on the 2nd Floor, so maybe he went straightaway, as they left the classroom to go look at the other projects, maybe his group walked West toward the stairs and Kyron went East toward the restrooms. Once he is finished, he thinks his group has gone downstairs to see the electric project and heads down the West Stairs, where someone grabs him from there and out the West Doors.
I think that the 2nd scenario seems more plausible, in some aspects, since the sub/volunteer would then know that Kyron was there to begin with, to note him missing afterwards. It does not, however, jive with published statements that TH was the last known person to have seen Kyron before he disappeared, which makes the 1st scenario more plausible.
I cropped and pasted the floor plans so that I could get a better visual. I'll post it here, in case anyone else wants to see it.