The lesson was very timely, Ms Johnsen said.
"It's the start of the season now that will run for a few months," she said. "You can actually have an epidemic go through a classroom if you're not careful."
A Northland District Health Board spokeswoman said although the exact timing of the flu season changed from year to year, it was usually between early June and September. However, because of the warm weather it was expected the flu season would start later this year.
The publicly funded influenza vaccine was usually available in March but was three weeks late this year. That was because the vaccine had to be changed to protect against two extra strains of influenza.
Te Tai Tokerau Primary Health Organisation population health strategist/analyst Catherine Turner said 11,285 people over 65 years, or 40 per cent, had been immunised in Northland by the end of April.
Because people over-65 years were the main target of immunisations there was no data available at this early stage for the rest of the population, Ms Turner said. That data would be available later in the season.
The rate for over-65s was slightly higher than previous years with the target being 65 per cent, Ms Turner said.
The lessons at Hurupaki School were part of a programme by Kleenex Tissues developed in 2005. The aim was to help teachers teach children why and how they need to trap their sneezes, bin their tissues and wash their hands.