Investigators said Holly Polson, 35, met the teen at Shadow Hills Intermediate School in Palmdale.
Sgt. Brian Hudson of the Los Angeles County Sheriff's Department said Polson was supposed to be tutoring the 13 year old after school. Instead, Polson was having sex with the boy on campus, he said.
"We entrust our children to teachers and school instructors and in this case that trust was violated," Hudson said.
Hudson said the boy's parents found inappropriate text messages that Polson had allegedly sent their son on his cell phone.
"Deputies contacted the victim and he disclosed that he had been having a sexual relationship with the suspect," Hudson said.
The victim told authorities that he started having sex with Polson two weeks ago. When investigators interviewed Polson, she allegedly admitted to the sexual relationship with the student.
"She admitted to several instances of having sexual intercourse with the victim on school grounds," Hudson said.
Neighbors near Polson's Palmdale residence said she lived with her husband and four children.
"My little brother used to play with their boys, so he knew them pretty well," neighbor Daniel Rocha said. "They seemed like such a great family. I'm really surprised. I would have never expected this."
Polson was arrested at her home Tuesday afternoon and booked on four felony counts of lewd acts with a child. Her bail is set at $100,000. Her first court appearance is scheduled for Thursday.
The boy's name was not released because he is a minor.