The Government has warned pupils caught expressing homophobic views could be at risk of extremism as it unveiled new plans in a bid to prevent a repeat of Birmingham's Trojan Horse scandal.
Education Secretary Nicky Morgan has issued schools across the country with a guide to identifying pupils in danger of radicalisation.
She said attacking core British values or being extremely intolerant of homosexuality were examples of behaviour that could raise the alarm.
The advice is designed to enable teachers to identify pupils who may have come under the influence of extremist organisations or are being "groomed" by extremists.
It comes after Ofsted last April plunged five Birmingham schools into special measures following an alleged plot by hardline Muslims to take control of governing bodies.
Ms Morgan told the BBC: "Radicalisation or the risk of children being drawn into non-violent extremism is a very real threat in this country."
She said the advice - which will apply to all schools, including independents and academies - would ensure teachers were better able to identify children who had become radicalised by changes in their behaviour or attitudes or remarks they made.
Asked to come up with an example of behaviour that might be a cause for concern, she said: "Sadly, Isis are extremely intolerant of homosexuality."
She was asked whether a pupil who said they thought homosexuality was "evil" would be reported to the police.
Ms Morgan said it would "depend very much on the context of the discussion" and schools were capable of using their judgement.
It comes after parents in Birmingham have reacted angrily to a new scheme being rolled out in local authority-controlled schools aimed at teaching pupils about lesbian, gay, bisexual and transgender (LGBT) issues.
In March, parents of pupils at Clifton Primary School, in Balsall Heath, signed a petition lobbying the school to put a stop to plans to implement the Challenging Homophobia in Primary Schools (CHIPS) programme.
They said the subject "conflicted" with the religious beliefs of the children, who are predominantly Muslim, while they also believed four year olds were too young to be taught the subject.
And last year, at Welford School in Handsworth, headteacher Jamie Barry was confronted by more than 100 angry parents worried about the lessons.
CHIPS and a similar scheme called Pride in Education (PIE) are due to be rolled out all Birmingham City Council-run primary and secondary schools by 2018.
They use books, music and videos to explain to pupils about children who grow up in all types of families - including those with LGBT parents.