Schools in a Virginia county closed on Friday after an outcry when students were asked to practise Arabic calligraphy using a Muslim statement of faith.
The high school geography assignment sparked a furious response from some parents and threats from outside the district.
Augusta County Public Schools officials said no specific threat of harm had been made against students in the area, about 240km southwest of Washington.
But the volume of calls and emails related to the lesson and concerns over their "tone and content" prompted the school closures, according to a statement on the district's website.
A week ago, students at Riverheads High School were studying the Middle East and were asked to trace a piece of Arabic calligraphy that translated to: "There is no God but Allah and Muhammad is the messenger of Allah".
It was part of a lesson that explained that calligraphy is sacred to Muslims, who decorate mosques with writing, rather than images of people or animals, since their religion forbids idolatry.
The assignment aimed to show the "artistic complexity of calligraphy".
Some pupils refused the assignment and parents accused the teacher of indoctrination.
Officials with the Augusta County school district were quick to point out that the study of a region's religion and language are included in geography lessons.
However, anger built up and by Wednesday there were so many complaints that the school's doors were locked and monitored.
By Friday, the messages - described as profane and hateful - had increased and the decision was made to close the schools.
Reaction follows San Bernadino shootings
The public outcry appeared to reflect the current mood of fear and distrust of Muslims among some Americans.
It follows the killing of 14 people in San Bernardino earlier this month, by a married couple said to be inspired by extremist Islamic beliefs.
The California shootings came just weeks after 130 people were killed in Paris by Islamic State fighters.
The school district statement said: "We regret having to take this action, but we are doing so based on the recommendations of law enforcement and the Augusta County School Board out of an abundance of caution."
Extracurricular activities on Thursday and on the weekend were also cancelled for the district's approximately 10,000 students.
Some parents have accused the teacher, Cheryl LaPorte, of trying to indoctrinate students with Islam and are calling for her to be fired.
"I do not trust her to teach my son and regardless of the outcome he will not sit in her classroom," mother Kimberly Herndon said in a Facebook post that has been shared more than 300 times.
The school district said in its statement that students would continue to learn about world religions as required by the state but a non-religious sample of Arabic writing would be used in the future.
"No lesson was designed to promote a religious viewpoint or change any student's religious belief," the statement said.
Ms LaPorte said she had received overwhelming support from former students, colleagues and others in the community.
"All I want now is time for our community to heal," she said in an email to Reuters.
In an unrelated closure, Franklin Community Schools in Indiana were shut on Friday after a high school was evacuated a day earlier due to a threat.
- Reuters, BBC