Teachers are being warned to look out for "tell-tale" signs of racist or extremist views among pupils after a series of disturbing classroom incidents.
The advice is part of new guidance being drawn up by the Assembly government on how schools can deal with threats to community cohesion.
It follows concerns about the growing influence of Muslim extremists, animal rights activists and groups like the Welsh Defence League, who are trying to recruit pupils to their cause.
There have been a number of serious incidents in schools in the last two years, which were revealed for the first time by headteacher Steve Bowden at last week's ASCL Cymru conference. These include:
- distribution of anti-Semitic literature to pupils;
- a Year 11 pupil who set up a website promoting far-right views;
- a pupil who went to school dressed as a jihadi on World Book Day;
- a pupil who said it was the duty of true Muslims to prepare for jihad and praised the 7 July bombers in London as "martyrs";
- a primary school circle time during which a number of pupils said they had been involved in physical attacks on children outside school to "make them go back to their own country";
- a supply teacher who deliberately left a book on show promoting extremist views.
Mr Bowden, who worked on the new guidance and is on secondment to the Assembly government from Porth County Community School in Rhondda Cynon Taff, told heads at the ASCL Cymru conference that they had a role to play in tackling the problem.
He said teachers must be on the lookout for signs of extremism such as racist graffiti or offensive views expressed by pupils. "You can't ignore these tell-tale signs - that will be every teacher's responsibility."
Heads must also be careful when appointing staff and be aware that some teachers could have "harmful influences" on pupils, he warned.
Job applicants should be thoroughly vetted and their CV rigorously checked.
Mr Bowden said heads must be effective leaders and empower their teachers to address these issues as soon as they arise.
He said: "There are currently a significant number of people across Wales who are being watched very carefully by the authorities - some of them in schools.
"We need to consider our response to this whole issue from a school point of view.
"Learning and education is the long-term solution to addressing these community cohesion issues."
The Assembly government has written to education directors in all 22 local authorities in Wales reminding them that they must have a dedicated person to whom schools can report such issues.
The new guidance will be issued to schools in January before a series of conferences in February.