But not all of the children will have displayed symptoms, highlighting the fact that the current pandemic appears to be of a mild strain of flu.
Across the rest of the UK, it is estimated that a fifth of children will have been infected with swine flu.
The latest findings come from a HPA study of blood sera tests carried out among 500 school children in England.
Professor Maria Zambon, director of the HPA's centre for infections, said that swine flu was having a limited impact on the older population because the over-50s have some degree of pre-existing immunity to the virus.
More information about swine flu on our online resource centre
