Based on facts and rumors, I am guessing this is possibly what might have happened.
"The girl" left the school at 3pm. She might have driven either to Target, and then to the boat dock. Or, she went to the boat dock, before Target...perhaps she didn't go to Target at all. Anyway, she possibly met someone, at the boat dock, and he joined up with her, around 3:40. He got in her car...and maybe, forcibly, against her will. She may, or may not, have known him, well. They possibly drove through a car wash. They drove around town, a bit, (and possibly went to the spec house but left, due to likelihood there would be no privacy.) There might have had an intense conversation. Either the girl, or her passenger, decided to drive out to CB, around 5. Once there, their intense conversation grew angry, and evil, leading to the girl's death. In an effort to hide the girl's body, Passenger-Killer took the body down to the bushes. Passenger Killer was in a rush and but he possibly had plans to return, and deal with the situation, later. He knew he had to return the girl's car, quickly. He cleaned up in the river, and left...with crime scene items-tools in a bag. He possibly drove the girl's car up to PC, and left it. He possibly hiked down the hill, and maybe rode the city bus, part of the way, to the boat dock, to pick up his vehicle. Then, maybe he returned something to the spec house, or he went back there to pick up something he accidentally left, there. oOr, maybe he didn't even return there at all. If he and the girl went there prior to going to CB, he might not have gone back there, after getting back from CB.
My ideas and thoughts are likely to change as I hear or read more informational facts and informational rumors.
I am still guessing the killer is a white male who acted, alone. I don't have any idea what he looks like, or what his name might be.
All the above is just a guess. it's not factual. I hope something is proven, soon, and Killer is arrested.