A Livermore high school math teacher who was arrested on suspicion of sexually assaulting one of her former students when he was 14 years old is expected to be arraigned today, authorities said Thursday.

Marie Johnson, 40, was arrested without incident at 7:24 a.m. Wednesday when she arrived for work at Granada High School at 400 Wall St.

Officer Steve Goard said detectives had worked through the night to wrap up their case against her.

Goard said police allege that Johnson began having sex with the victim, a boy who was 14 years old at the time, shortly after he finished

taking a class from her in December 2010 and sexually assaulted the boy 24

times over a six-month period.

He said investigators don't know why the alleged sexual assaults ended in May but it could have been because that's when the school year ended.

Police believe the alleged sexual assaults occurred in Johnson's car and in other undisclosed locations in Livermore, according to Goard.

Investigators believe Johnson and the boy, who is now 15, formed their relationship through text messages, social networking on Facebook and

instant messaging on a smart phone application called "Words with Friends," Goard said.

He said police don't think there are any other victims but they recommend that parents speak with their children about the nature of any

contacts with Johnson.

Johnson, a Livermore resident who's being held at the Alameda County Jail in lieu of $1.85 million bail, tentatively is scheduled to be arraigned in Superior Court in Pleasanton at 11 a.m. today.

Goard said investigators are still completing their report on her case and the District Attorney's Office might not file charges against her until early today.

Livermore Valley Joint Unified School District Superintendent Kelly Bowers said in a statement that Johnson has been placed on administrative leave by the district "for alleged inappropriate relations with a minor, pending investigation of the allegations."