The World Health Organisation has warned that a pandemic is "imminent", and concerns have been heightened by the spread of a severe form of avian influenza (H5N1) among birds in Asia.
Closures could help reduce the spread of flu among children. But teachers will be expected to carry on working from school unless they are unwell.
And they may have to provide pupils with schoolwork by post or online if schools are closed for more than two weeks, or to work in other schools to keep services going.
Even if schools are not directed to close by their local authority, some may be forced to do so if too many staff fall ill. Local education authorities and school governors are being told to "positively" discourage people from coming to work who are unwell.
New Assembly government guidance issued to schools, LEAs, children's homes and childminders warns that at least 2,800 people could die in Wales during a flu pandemic, with a quarter of the population expected to fall ill.
Headteachers and LEAs are being urged to start planning now for emergency closures directed by the Assembly government.
But heads and governors also need to work out at what level of understaffing they would have to close schools. Staff may be asked to work flexibly, and pupils could be regrouped or supervised by unqualified volunteers.
Schools will need to ensure "rigorous" hygiene measures - including a plentiful supply of hot water, soap, tissues and tissue disposal.
Managers will need up-to-date contacts for staff and parents, including email and mobile numbers, and procedures for dealing with the death of a pupil or member of staff.