A year 12 student at a Sydney high school is being investigated by counter-terrorism police over allegations radical Islam is being preached in the schoolyard.

The 17-year-old boy attends Epping Boys High School in the city's north-west.

Police have not confirmed details of the investigation but said the radicalisation of young people, including school students, was a worsening problem.

Epping Boys High School principal Tim O'Brien said school counsellors were available for students.

In a statement on the school's website, he said the school was in close liaison with the Department of Education and law enforcement agencies "to uphold our exemplary levels of student safety and student well-being".

"It is important I emphasise that student safety, well-being and support is our absolute highest priority at all times," he said.

"School counsellors are available for all boys, if required, today or in the future.

"All normal lessons and activities are proceeding today according to timetable."

News Corp reported the investigation was looking at possible links between the boy and Milad bin Ahmad-Shah al-Ahmadzai, who was last year convicted of threatening to slit the throat of a Commonwealth official.

Al-Ahmadzai is currently in police custody charged over a number of other violent crimes in Sydney.

One mother said she had previously raised concerns about a student talking about the Islamic State group. ( ABC News )

Outside the school this morning, one mother said she had previously raised concerns about extremism with the school.

"We really had [a] problem with [a] student in the school and we talked [to] the principal and everybody, but now everything is fine," she said.

"Some student encouraged another student. They [were] talking about the ISIS group."

The woman, who has two sons, aged 18 and 14, at the school, said she spoke to the principal about a month ago.

"The principal said, I think, he talked with another family and student, and he's a really, really good principal with everybody."

Ages of people exposed to radical ideologies falling: police

NSW Police assistant commissioner Mark Murdoch, who heads the Counter-Terrorism and Special Tactics Command, said parents needed to be reassured police and schools were working together to protect students.

"This school that is the current focus of our attention is not unique," he said.

"We are in the business of keeping students and community safe right across the state, not just focused on one particular school or one particular set of circumstances."

He said the ages of those becoming exposed to radical ideologies was falling.

"We are no longer dealing with people in their 30s but we're seeing people of much younger ages becoming involved," he said.

"So it stands to reason that any of these young people would be in the education system or may have recently been at school.

"But please, people need to be assured that we have mechanisms in place, in conjunction with the education department, to deal with the problem."

The State Opposition's education spokeswoman, Linda Burney, said the threat of schoolkids being exposed to radical ideologies at school was deeply troubling.

"There is a pressing need to have a statewide and very specific response developed with the community leaders and other government agencies on issues such as this," she said.

"Schools can't be expected to deal with these sorts of issues on their own."

In a statement, the State Government said it was working with community leaders and experts to develop programs to counter violent extremism.