With my eyes the way they are, I seem to need a break before I get completely caught up on here, and get pages behind again by the time I come back. Because of that, I've been skimming through many posts instead of reading each one word for word like I usually do. Please forgive me if this has already been suggested by someone else (my ideas are usually so off-the-wall that most seem to be unique though!)
It seems obvious (to me) that the perp wasn't too concerned about hiding the identity of the body, or he would probably have also hidden the backpack somewhere. I was wondering last night if there could be another "practical" reason for dismembering a body, and I did think of one today. It's possible that the person isn't physically capable of carrying 60+ pounds; it could be because he/she is small, has a bad back, some other health problem that restricts lifting (heart, lungs, etc.) or maybe an injury that requires crutches or a cane, which would make it somewhat necessary to use just one hand for carrying.
I have no idea who the neighbors are, and there are probably none that fit, but I was thinking that someone could have asked her for help carrying something into the house, and offering her and her friend a ride to school if it took too long. That could get her inside without having to carry her, but then getting her back out would be a problem - it would have been too obvious to be seen dragging her out, even if she was wrapped in something.
MOO