if roger was taken off school grounds not under his own power then that would have been impossible given the many witnesses.
the only thing I can think of is that if the school had a secluded location he was lured their and killed. But that would mean the murder would have had to have been done quickly, because the other students and faculty would have realized whomever done it was missing. And if pash was indeed involved I don’t think he acted alone, I think other students at the school know more, this seems like a cover up by the entire school. Roger saw something possibly that was not supposed to happen, perhaps whatever he saw would make someone lose their job or get fired so he was silenced, and perhaps pash inviting Ellison to his house was to try to get him to not tell on him, for whatever he was doing maybe he tried to talk Ellison out of it, if he tried to form a close relationship with him in order to get him to change his mind and not tell.