Schools in Augusta County, Virginia, closed on Friday as a safety precaution after a homework assignment asking students to copy Islamic religious calligraphy prompted an angry backlash from parents and threats from outside the district.

County school officials did not point to any specific threat of harm but said on the school's website they were canceling Thursday extracurricular activities, adding that classes would be called off the following day.

The assignment, handed out to Riverheads High School in Staunton, was part of a broader unit on major world religions and asked students to copy Islamic religious calligraphy.

County school officials did not point to any specific threat of harm Thursday afternoon they canceled extracurricular activities as well.

After objections from parents and media coverage, the schools received "voluminous phone calls and electronic mail locally and from outside the area," Augusta County Public Schools said in a statement.

Police presence at schools was bolstered, but the communications increased. Because of concerns about their tone and content, officials decided to close schools and school offices on Friday "out of an abundance of caution," the statement said.

"No lesson was designed to promote a religious viewpoint or change any student's religious belief," the statement said. "Although students will continue to learn about world religions as required ... a different, non-religious sample of Arabic calligraphy will be used in the future."

Augusta County is about a 170-mile drive from Washington.

A rash of email and phone threats of violence hit schools across the country this week, but most were deemed to be hoaxes and schools opened.

However, three suburban Indianapolis school districts reported potentially serious threats, with two districts shutting down on Thursday and another canceling classes for Friday.

Officials have been on heightened alert after the deadly attacks in San Bernardino, California, on Dec. 2. On Tuesday, Los Angeles shut down public schools because of emailed threats that later were deemed a hoax.