Nearly 1,000 friends, classmates, teachers and neighbors attended the vigil.
Jenna's friends and family lit candles and shared stories telling each other what she meant to each of them and what a tragic loss her death is.
Jenna was hit around 5 p.m. Sunday on the railroad tracks near Old Orchard and Howe roads. The Burlington Northern Santa Fe train that struck her was heading from Richmond to Fresno, and had three crewmembers aboard at the time of the incident, a railway spokeswoman said.
The spokeswoman said Jenna and another person were on the tracks but moved away as the train approached. The girl then returned to the tracks and was hit, she said.
According to a Facebook message from Jenna's mother, Dena Betti, that was reposted on social media sites, Jenna was with her boyfriend on the tracks just before she was hit. The mother wrote that Jenna apparently dropped her phone and was killed when she went back to grab it as the train approached.
Grief counselors have been at the school and will remain there all this week, the secretary said.
On social media, messages about Jenna have been pouring in since her death. One poster, who appears to be Jenna's boyfriend, wrote, "She is so important to me and I am just devastated. I love her so much."
Jenna has two younger sisters and played club soccer. Her mother was involved in her daughters' school communities, according to the Martinez Unified School District superintendent.
ABC7 News contributed to this story.