If she slept at home that night, I don't know how or why he'd leave with her in the morning and return, to dump her items.
Is it possible he created a chain of deception?
For example, she does to talk to her mom, by phone/face on her way home from her party but never makes it inside. SS tells Mom she was so, so tired she went straight to bed, and Mom has no reason to question/test it. Believed MS was asleep in her room all night.
The next morning, he had to get her backpack -- was Mom home? at work? Asleep? Awake? Did he say he was grabbing her backpack and she was already in the car? Things that would seem believable -- so having her in the car was a necessary 'proof of life' prop in order to give himself the day for concealment, he just needed to manipulate Mom into thinking he'd brought MS home from the party, picked her up and there she is, already waiting in the car....
Accomplished gaslighter. Fool tried to gaslight the media too. Taking the blame for the lesser sin -- as if something bad happened to MS after he dropped her off, short of school (as if he had). Smokescreen for what he did before the imaginary drop off. Made a grotesque play for sympathy.
Breaks my heart to think of all the mind bending he's been doing.
Nothing short of evil.
Jmo