I do wonder what the issue was with the person she was having problems with, and if a male was involved in some way; with this age-group, there almost always is. This could be one of those scenarios like the so-called Cadet Murder, where the boy and his girlfriend adbucted and killed the girl that the guy had cheated with, to make it right with his girlfriend, for instance, (Diane-what was her last name?) She was hiding in the backseat when they abducted Audrey and took her to a field and killed her. I could even see the handcuffs coming into play, in that kind of a scene...in that case, Audrey got into the car with the guy (David?) since she knew and liked him...
Anyway, that crime came to mind tonight as a possible scenario. Also there was the recent murder of Michele Le, presumably by the woman in custody, all over a man...the most common (and dumbest, IMO) reason of all...