Summer Michelle Hansen was arrested on suspicion of engaging in unlawful sex with a minor and oral copulation of a minor, and has been put on leave from her job while officials investigate the allegations.
Mums and dads with kids at Centennial High School in Corona, California, have been told to ask their children if they have had any unwarranted contact with the married 31-year-old.
The police believe Hansen had sexual relations with 'multiple' youngsters, but admit they do not know exactly how many.
Her behaviour came to light when a former student claimed he'd had sex with her, resulting in the school informing the police.
Investigating officer superintendent Tom Pike said the relevant security screenings had been in place when Hansen was employed.
"All teachers undergo a thorough background check prior to employment, including clearance from the FBI and Department of Justice," he told ABC 7.
The Mail reports that parents at the school were 'stunned' by Hansen's arrest.
"It surprises me that any 31-year-old adult, married woman is having sex with a child, teacher or otherwise,' said one mum, Angela Thompson, while another said Hansen should be given the opportunity to tell her side of the story.
"Give her fair chance to defend herself, and if that is the case it's going to be a shame," parent Khoi Pham said.
The Los Angeles Local news site reports that Hansen's Facebook page reveals she has been a teacher at Centennial since 2008. It also claims that 'Bad Teacher' starring Cameron Diaz is listed as her favorite movie, and 'To Catch a Predator' her favorite television show, although friends of the teacher say she has been falsely accused, and that her Facebook page been hacked.