The UK's Education Secretary has condemned the "threats and intimidation" a teacher faced after he was suspended for showing pupils satirical cartoons of the Prophet Muhammad.
It comes as videos on social media showed people gathering to protest outside a West Yorkshire school on Thursday, with some calling for the teacher to be sacked.
Batley Grammar School advised that students should stay home and police officers were put on guard outside.
Gavin Williamson labelled demonstration "completely unacceptable", and said teachers were within their rights to expose pupils to “challenging or controversial” issues.
"It is never acceptable to threaten or intimidate teachers. We encourage dialogue between parents and schools when issues emerge," he said in a statement late on Thursday evening.
"However, the nature of protest we have seen, including issuing threats and in violation of coronavirus restrictions are completely unacceptable and must be brought to an end.
“Schools are free to include a full range of issues, ideas and materials in their curriculum, including where they are challenging or controversial, subject to their obligations to ensure political balance.
"They must balance this with the need to promote respect and tolerance between people of different faiths and beliefs, including in deciding which materials to use in the classroom.”
The grammar school's headteacher, Gary Kibble, apologised "unequivocally" to parents in a statement for the fact that caricatures from the French satirical newspaper Charlie Hebdo had been shown to students during a religious studies lesson earlier in the week.
He said the teacher in question had "given their most sincere apologies" and had been suspended with an investigation into the incident to be opened.
Representations of the Prophet Muhammad are highly offensive to Muslims.
In 2015, 12 people were killed by two gunmen in an attack at Charlie Hebdo's offices in Paris after the magazine printed caricatures of Prophet Muhammad.
French history teacher Samuel Paty was beheaded in late 2020 outside his school near Paris after showing the Charlie Hebdo cartoons during lessons on freedom of expression.
The classes led to complaints by parents and an online campaign against Paty, which prosecutors say has a "direct causal link" to his murder.
An 18-year-old radicalised Chechen refugee was shot dead by police outside the school in Conflans-Sainte-Honorine. He had earlier offered students money for identifying the teacher before attacking him.
The assassination stunned France and led to an outpouring of support at memorial ceremonies and marches around the country.
Fourteen people, including six students, have been charged in the investigation.
In the wake of Paty's beheading, French President Emmanuel Macron has promised a further crackdown on extremism, dissolving the "Cheikh Yassine Collective" for its "direct" implication in the terror attack and ordering a mosque in the northeast Paris suburb of Pantin to close for six months.
But some critics said the government's measures to handle the situation are disproportionate and dangerous.
Leading the condemnation was President Recep Tayyip Erdogan of Turkey, saying Macron "needs mental treatment" in a speech. “What is the problem of this person Macron with Muslims and Islam?” he added.
Macron's actions also sparked protests in the Middle East and North Africa, especially in France’s former colonies, with tens of thousands turning out in some places and burning effigies of the French president.