Okay going over this in my head.
To me suicide is out, no one is going to pack and throw away their purse/bag if they are going to commit suicide.
I also thought of the changing into something at school and then changing back mindset, but you would not have the panties/bra packed in that case.
The phone was obviously thrown out immediately, all new/police reports lead to that by the mud and wetness. But who is to say the purse/bag was not thrown out later if nothing else to lead away from the actual crime scene.
I have a bad feeling the clothing in the bag were what she was wearing that day, I think whatever/whomever made her change or made her remove them. If they folded them neatly to dispose of them in her bag, they knew her well and cared enough to fold them afterwards. It was someone who cared for her.
I know the dogs hit on the scent leaving the house and to the driveway, I am sorry that their are alibi's for that morning, but I am leaning towards thinking something happened in or around the house. If she was packing for a secret overnight, she would have packed make up/phone charger/brush and taken her inhaler. IMHO
And to add, I also think that is the strap of her bag in the photo above on twitter, that is not her bookbag, she obviously loved that bag. Would love to know if they found her books as mentioned above or not, would like some clarity on that. I believe this was her purse.