The Flusurvey surveillance project is in its fifth year, but for the first time classrooms are being asked to take part.
It coincides with the NHS beginning to roll out childhood flu vaccinations. In time, the nasal spray vaccine will be available for children aged 16 and under.
Key spreaders of flu
Head of the project Dr Alma Adler said: "Last year we found that taking public transport does not increase your risk of catching flu and we discovered that 'man flu' didn't really exist - in fact women were slightly more likely to report feeling worse when they have flu than men.
"This year we're keen to find out more about children because they are the 'key spreaders' of flu."
This flu season it is being offered to all children aged two and three, as well as children of any age with long-term health conditions such as diabetes.
In Scotland and seven pilot areas in England, children aged four to 11 will also be eligible for it this year.
In order to help control and lower the impact of flu, the Flusurvey researchers from the London School of Hygiene & Tropical Medicine want to understand what role children play in catching and spreading the disease.
The Flusurvey team will be working with the British Science Association to link up with schools for the 2013-14 flu season to monitor the impact of the virus. In 2012-13, the highest rates of infection were reported in the under-18 age group.