I am reading through early reports on KTVU, trying to pinpoint when the timeline changed from sierra leaving the home with her mother in it at 6am, to marlene leaving first at 6am and sierra an hour and a bit later. Did the timeline change around the time the RSO info about her father came out? Sometihng distracting must have been happening.
"She was in an optimistic mood," Marlene Lamar said about her daughter. "She was very cheerful. She wished me a good day at work. I told her I loved her before she left for school." implies to me Sierra left first, else you'd say "before/as I left for work".
From this article : http://www.ktvu.com/ap/ap/california/searchers-for-missing-norcal-teen-found-cellphone/nLXjT/ dated March 19th. All references to Sierra leaving before dawn at 6am are attributed but not directly quoted from Marlene.
Looking for when that changes ...
This also makes it appear Sierra left first. I don't think she did but it looks like they were given information she did.
"Sierra's family reported her missing Friday afternoon after last seeing her around 6 a.m. when was she heading off to Ann Sobrato High School, Cardoza said."