With Americans already on edge after the attacks in Paris and Southern California, a teacher's lesson on Islam in a Virginia school sparked an angry meeting of outraged parents that mushroomed into a national denunciation of the educator in the form of thousands of angry emails and social media postings.

As a result, an estimated 10,000 students in Augusta County's public school system got a one-day jump on the Christmas break as a precaution. The cancellation of classes Friday also wiped out a holiday concert and weekend sporting events.

Some of the tens of thousands of emails and Facebook posts "posed a risk of harm to school officials" and threatened protests, Superintendent Eric Bond said in a message to parents and employees Friday.

Augusta County Sheriff Randall D. Fisher said the emails slowly began arriving, but picked up in volume and vitriol after a national conservative radio personality discussed the lesson.

"They started becoming very threatening, very profane," he said. "There was a lot of hate being spewed in these emails."

Some, Fisher said, had images of beheadings.

Fisher said security has been assigned to Bond, the school's principal and teacher Cheryl LaPorte, who created the lesson. LaPorte is worried about herself and her family, Fisher said.

The school closings came one week after the teacher gave Riverheads High School students an assignment that involved practicing calligraphy and writing a statement in Arabic — the Shahada, a profession of faith recited in Muslims' daily prayers.

The statement translated to: "There is no god but Allah and Muhammad is the messenger of Allah." The teacher's lesson was drawn from instructional material that also includes Judeo-Christian assignments.

School officials said the aim of the lesson was to illustrate the complexity of the written Arabic language, not to promote any religious system, and a different sample text will be used in the future.

But some in this deeply religious area in the foothills of the Blue Ridge Mountains said the lesson angered them, especially because of what they said were efforts to marginalize Christianity in the schools.

Jackie Duff, a Baptist, said LaPorte should have been mindful of the slayings of 14 people in San Bernardino this month by a Muslim couple who had become radicalized.

"I think their wanting to take prayer away from school, their wanting to take God out of textbooks, that was really like a slap in the face," Duff said over dinner at a fast-food restaurant outside Staunton.

She did not agree, however, with the hateful emails directed at LaPorte.

"I don't think that they should be attacking her in that manner," Duff said.

At a forum Tuesday, one parent said the assignment promoted a false religious doctrine, while other parents expressed outrage. Some demanded the teacher be fired.

LaPorte declined comment. A Facebook group supporting her had more than 2,000 members Friday. Many in this Shenandoah Valley community defended LaPorte and the school district.

"I think people are making a big deal about it for no reason at all," said 18-year-old Hannah Carey, a former student of LaPorte who lives in Waynesboro. "We learned about all different cultures, and she was a great teacher."

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Associated Press reporters Alanna Durkin and Alan Suderman contributed to this report from Richmond.