A Petaluma high school teacher was arrested Saturday morning after allegedly setting up an illicit date with a person he believed was a 15-year-old girl, police and prosecutors said.

Scott Eugene Dietlin, 34, a teacher at Casa Grande High School, was arrested in San Mateo on Saturday after allegedly showing up for a date with “Jackie,” a teenage persona created on MySpace by two teen boys, San Mateo County Chief Deputy District Attorney Steve Wagstaffe said.

The 15-year-old San Mateo County boys posted the online profile as a joke, Wagstaffe said. When Dietlin allegedly contacted “Jackie” in April, the boys initially played along, he said.

“They were playing a joke with their friends and suddenly they’ve got a sexual predator,” Wagstaffe said.

Over time, the communication included sexually explicit e-mails and text messages, police said. At one point, Dietlin allegedly sent “Jackie” a naked photo of himself, Wagstaffe said.

Contact faded but resumed in recent weeks and the boys finally decided to call police, Wagstaffe said.

Burlingame police began posing as “Jackie” and set up a date at a location in San Mateo on Saturday, Wagstaffe said.

Dietlin showed up that morning and was taken into custody, police said. His bail has been set at $550,000.

Burlingame police searched Dietlin’s car and Petaluma home, seizing items including condoms, a computer and a cell phone, police said.

Dietlin, who teaches world history and economics, has worked at Casa Grande High School for nine years, Superintendent Greta Viguie said.

A substitute teacher has taken over his classes and counselors are on hand at the school to speak with any students distressed by the allegations, Viguie said.

“We recognize that this is a difficult time … and our focus is on the students and educational programs and ensuring that we continue to provide a high-quality educational experience for our students,” Viguie said.

A letter is being sent home to parents today, she said.

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