An entire year group at Whangārei Boys High School is at home today because too many teachers are sick.Influenza A is currently hitting the country hard - the past couple of weeks have been the worst in three years for illness-related school absences.In a Facebook post on Wednesday, Whangārei Boys announced that all of its Year 10 students would not come into school on Thursday due to staff and relief teacher sickness."Due to a significant number of staff and relief teachers being unwell, we're unable to safely staff all classes."Year 10 students will be rostered home for the day - all other year levels attend as normal. Our library will be open and supervised for any Year 10 student who can't stay home; they should report there on arrival."The school said Year 10s would be able to return to school on Friday.Meanwhile, a group representing 112 Auckland kindergartens says at some centres, more than half of the children are at home sick this week.Flu-related hospitalisations are at a 10-year high in Auckland, with reports of under-fives in intensive care.Starship Children's Hospital is deferring less urgent cases as it struggles to cope with a "flood" of very young children in hospital with flu symptoms.Leanne Marshall, who oversees the Auckland Kindergarten Association's north-west centres, says one of their kindys normally has a roll of 40 children, but yesterday they only had 17.She says anecdotally, students and staff are dealing with flu-like symptoms for longer this year."The recovery time seems to be a lot longer than normal; two to three weeks. Some students and teachers are being away for that length of time, which is a lot longer than we would normally see."Marshall says they're asking parents to keep sick children home to prevent the flu from spreading.Doctors say the free under-fives flu vaccine announced for next year cannot come soon enough and will save lives.