We've been through this before, but, again, I find the Clark girls' sighting much more credible as to time frame than the others you mentioned. For one thing, the JCB statement was under oath whereas the other statements were merely responses to police questioning. In the statement, JCB didn't say that she saw the boys playing in the back yard at 5:30, but in the 5:30 -6:30 time frame. Also, with the boys on bikes and the relatively small area we're discussing, IMO, there's simply no contradiction. Besides, the more important time, to me, is the 6:30 time.
As to RC being seen by JCB at school, as I've repeatedly explained, he could have gone by the school shortly after the bodies were discovered and seen JCB in the school yard. She didn't say she saw him in school, but rather at school. IMO, there's a difference.
Here's a possible scenario. The bodies are discovered and the parents notified. This would have been around 2 pm or 2:30 pm, IIRC, by the time all three bodies were found. JMB and his wife (MB) are distraught. RC could have been more or less "forgotten" at this time. He could have slipped away and gone by the school, seeking solace or for some other reason, like getting his assignments.
According to JCB's statement, she and RC were friends and walked home together from school every day. Although RC had not attended school that day, he could have gone by the school, either to seek solace from his friend or maybe to pick up assignments. Since RC attended a private school at the time, he would have been more likely to be diligent about assignments. When I was teaching, there were several times that a student came by to get assignments right after a traumatic event - and I taught at public schools!
So, either for one of the reasons I mentioned or for some other reason, RC went by the school and saw JCB. In her statement, JCB even mentions that school was dismissed right after she talked with RC. Unfortunately, she didn't clearly state if this was at the "normal" dismissal time (I'm assuming around 3 pm) or if school was dismissed early because the news of the bodies being found had been made public. Either way, however, RC's "dropping by" the school, IMO, is a distinct possibility. Yes, this is speculation on my part. However, I believe that the scenario I outlined is plausible, at least.