The accident was reported in the 1300 block of North Bronson Avenue at 7:48 a.m. The semi was going southbound on Bronson Avenue at the time of the crash.
Both victims were rushed to a local hospital, where the mother, who was in her 30s, succumbed to her injuries. Her name has not been released. Authorities say the girl suffered minor injuries and is expected to be OK.
The driver remained on scene to speak to police. Police say they had initially received conflicting witness statements on whether there was another vehicle involved in addition to the semi, but they believe only the semi truck was involved.
PJ Johnson, a parent, says she spoke briefly to the truck driver following the accident.
"He was like, 'Well she just stepped out on the walkway,' and I'm like, 'Well, that's what a walkway is for,'" she said.
The girl is a student at Citizens of the World Charter School, which is on the campus of Joseph Le Conte Middle School. A Los Angeles Unified School District official says grief counselors will be available for students and staff.
The cause of the accident has not been determined. The driver is not being cited at this time.
Some parents and residents say drivers are often going too fast in the area even though it is a school zone. Before and after school, the intersection of North Bronson and Fountain avenues is jammed with students crossing the street and drivers waiting their turn at the three-way stop.
The LAUSD says it requested a crossing guard in January 2013 for the intersection, but the Los Angeles Department of Transportation did not provide a guard.
Residents say a crossing guard and speed bumps are needed to help prevent accidents in the future.