Lessons cancelled in school in Virginia after parents complain children could be indoctrinated with Islamic beliefs.

With the US already on edge after the California attacks, officials have cancelled lessons on Arabic calligraphy – and shut schools early for winter break – after complaints from parents that their children could be indoctrinated with Islamic beliefs.

Administrators in Augusta county in the state of Virginia closed for winter break early on Friday after threats of violence.

Over 10,000 students were affected by the closures, which officials said they were forced into after receiving thousands of hostile emails and social media posts.

The calligraphy assignment that caused the outrage asked students of Riverheads High School to draw out the Islamic declaration of faith – or Shahada – as part of a comparative religion class.

How one Arabic calligraphy lesson led to the shut down of an entire Virginia school district: https://t.co/jCNOeYPHqs — AJ (@ajplus) December 19, 2015

The lesson was intended to help students appreciate the Arabic calligraphic style, often expressed in Islamic religious texts.

While some parents complained the class was trying to convert their children to Islam, others said the controversy was a symptom of anti-Muslim sentiment that was fuelled by the ongoing Republican presidential race.

During Tuesday’s Republican debate, candidates referred to “Islamic terrorism” more than a dozen times and mentioned ISIL almost 90 times.

Leading candidate Donald Trump has made no secret of his intention to ban Muslim immigration and close down mosques, should he be elected.

READ MORE: Arabic calligraphy fracas closes Virginia school district

Dalia Mogahed, of the Institute for Social Policy and Understanding and a former adviser to US President Barack Obama, says those complaining were acting out of fear.

“When we are being fed fear, day in and day out, we act as human beings, we act out of fear and out of irrationality, that’s just a symptom of a much bigger problem,” she told Al Jazeera.

“When our judgment is impaired, we’re not going to make a rational decision.”

Obama, the leading Democratic candidates and some of Trump’s fellow Republicans have rebuked the rising tide of anti-Muslim sentiment but the billionaire continues to rise in popularity.