At least 66 schools in England have suffered swine flu outbreaks since the start of term.
The number of cases across the country has almost doubled in a week, from an estimated 5,000 to 9,000.
Chief Medical Officer Sir Liam Donaldson said: "Everything suggests we are seeing the start of the second peak, but we just don't know how big that will be."
The number of deaths linked to the virus in the UK is 82 - up three in a week. The number being treated in hospital in England is up from 143 to 218 this week.