Thousands of students have been forced to stay at home after a geography assignment in which pupils were asked to write a Muslim prayer provoked a furious backlash.
Augusta County School District shut 23 schools in Virginia because they had received messages that were "threatening and very profane in nature", said police.
Sheriff Randall Fisher said security has been assigned to the district superintendent, Eric Bond, the teacher who set the lesson, and to the school principal.
A geography teacher at Riverheads High in Staunton, Virginia, gave her students the assignment one week ago to try out calligraphy and write some Arabic.
Students were asked to copy out the shahada, or Muslim statement of faith, which reads: "There is no god but Allah. Mohammed is the messenger of Allah."
Parents were angered by the assignment, with posts on Facebook accusing the school of religious indoctrination.
The school district's statement read: "Following parental objections to the World Geography curriculum and ensuing related media coverage, the school division began receiving voluminous phone calls and electronic mail locally and from outside the area.
"As a result of these communications, the Sheriff's Office and the school division co-ordinated to increase police presence at Augusta County schools and to monitor those communications.
"The communications have significantly increased in volume today and based on concerns regarding the tone and content of those communications, Sheriff Fisher and Dr Bond mutually decided schools and school offices will be closed on Friday, December 18."
In the statement, school officials insisted that "no lesson was designed to promote a religious viewpoint or change any student's religious belief".
It added that students would continue to learn about world religions as required by state academic standards, but would practise calligraphy using a different example.