The teachers is 41-year-old Heather Butts, who was arrested last week.
Officers had found Butts with the 15-year-old girl in a parked car on Santa Helena Avenue in San Bruno.
Butts taught English and drama at Taylor Middle School in Millbrae. ABC7 News is not releasing the girl's name or where she goes to school.
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Butts apparently had a long relationship with the girl. What's more disturbing is that ABC7 News has learned that a parent went to Millbrae police as far back as last October. That's ten months ago to file a police report after he discovered the illicit relationship.
Millbrae police interviewed the parent, his child and several other students. During that interview, they got screen shots of text messages and exchanges from social media sites between the teacher and the student.
The parent told ABC7 News that police assured him they would turn all of this over to the school district.
The parent, who does not want to be identified, said that in February, he also went to the school's dean of students about Butts' inappropriate behavior.
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He says the administrator told him the school district was aware of the police report. But were they? If so, why was Butts was still teaching there until San Bruno Police arrested her last week? And did the sheriff's office, which runs the Millbrae Police Department, even launch an investigation back in October when the police report was filed?
ABC7 News spoke to three retired educators about the story.
"Immediately act on it. That teacher should have been removed from the classroom," said Ernistine Forcum, a retired school principal.
"They definitely should have jumped on it as soon as it was reported," said Sophia Chester, a retired school teacher.
"Something is going on there. Covering it up for political reasons of some sort," suggested Kathleen Papperta retired school teacher.
San Bruno police say they now have Millbrae Police Department's report.
The school district's superintendent told ABC7 News reporter Vic that this is a "complicate case" and also a personal case, so they can't comment on it.
There are two police reports. One dates back to October and supplemental one that goes back to January, when parents went to the dean of students.
Both the sheriff's office and school district have launched internal investigations.
In an email to ABC7 news, the Sheriff's Office says that the two reports of a Millbrae teacher having a sexual relationship with an underage girl "were not investigated as expected" and were not "conducted in a timely manner." As of Friday, the Sheriff's Office has since started a formal administrative investigation into why the initial investigation was not completed.
"I'm disappointed the investigation was not completed in a timely manner and it does not reflect the level of service we routinely provide the residents of Millbrae or the communities we serve." said San Mateo County Sheriff Carlos Bolanos.
San Mateo Sheriff tells ABC7news they're conducting internal probe on why police report last Oct. re: Millbrae teacher was not completed.— Vic Lee (@vicleeabc7) August 27, 2016
ABC7news learns San Mateo Sheriff and Millbrae Schools knew about illicit teacher-student ties long before her arrest last week.— Vic Lee (@vicleeabc7) August 26, 2016
Teacher Heather Butts placed on leave while investigation into felony charges continues. Many questions on who knew and when?— Vic Lee (@vicleeabc7) August 27, 2016
Millbrae School district where teacher works says its "complicated" and can't comment because it's a personnel issue.— Vic Lee (@vicleeabc7) August 27, 2016