The teen was last seen Monday morning at San Ramon Valley High School, where she is a sophomore.
Her parents notified Danville police Monday night when she failed to return home. Police, family friends and fellow students launched an unsuccessful search in the neighborhood.
A family friend told ABC7 they believe the girl showed up for school for only a little while, then headed to the Golden Gate Bridge.
"We looked through her computer at home and found directions to BART in Dublin and then from the Embarcadero to the Golden Gate Bridge," Kevin Sawchek, whose daughter is close friends with Bayliss, said.
Sources close to the investigation say they also found a note on her Facebook profile which leads police to believe she may have gone to the bridge to take her own life.
When Sawchek was asked what kind of mood Bayliss was in when her parents last saw her, he said, "I think she was upset in a general sort of way and maybe feeling a little isolated is all I can say."
Bayliss' friends launched a "Find Alliy" page on Facebook, the goal of which was to recruit as many students to volunteer for the search, according to Ryan Levenson, a student at the school who helped spread word of the page.
"If we do find her," Levenson said, "that will be the biggest source that we used to find her. It helped to bring together or community and organize things quickly."
Police estimate more than 100 volunteers are now involved with the search.
Golden Gate Bridge police are reviewing video from security cameras mounted on the bridge. They are starting first with cameras closest to the sidewalks that lead to the bridge.