I dunno. There was't much mention of this party by SL. It seemed more just like a rumor going around and she didn't have any intentions of actually going to it. (Although I haven't read through the twitter posts in a few weeks).
Also, why would she need to cut school to go to the party? She had to have known the school would contact her mom about the absense. And she was planning to meet a friend before class to discuss homework according to her last text.
I really think the "project X party" has nothing to do with this.
I think she left her home intending on going to school and someone she was fairly well aquainted to picked her up and offered her a ride. IMO, this was either someone that has given her rides in the past, or someone she had spoken to in the area in the past.
The clothing in the bag could mean many things (depending on whether it was the clothes she was wearing, or a spare) and their are many possible explanations for both scenarios with the amount of information we have.
I believe SL's sister was coming into stay with her father in Fremont the night she went missing, but I never really heard any mention of family plans for that weekend.
I dont know how close she was to her sister, but I can imagine if she were going to a party in Livermore, she'd possibly invite the sister, or ask for a ride. Just my opinion.