An entire year group at Whangārei Boys High School is at home on Thursday because too many teachers are sick.

Influenza A is 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.