More than 20 Auckland secondary schools have been notified after a student has been found to have measles after attending a school ball on Saturday night.
St Peter's College headmaster James Bentley said the affected student was from another school and attended the St Peter's ball as the partner of a college student.
He has asked all St Peter's College parents to keep their sons at home during the contagious period from September 9-16 if they have not been vaccinated against measles.
He has also notified more than 20 other schools whose students were among the 464 students at the ball.
"It's been a pretty busy day contacting those schools. They are then using their own communication channels talking to their communities," he said.
The case comes on top of 778 confirmed cases of measles across Auckland so far this year.
Bentley said he was informed about the latest case by the school of the girl who attended the ball.
"A partner from another school has come down with measles this morning [Sunday]," he said.
"The [girl's] school had found out about it. They are still dealing with their community as well.
"The advice we have taken from the public health service is that the danger period where someone is not immunised is in one week's time. The disease is actually in incubation until then.
"So from next Saturday (September 9) until the following Saturday (September 16), no student who was at the ball and was not immunised is to come to school, because that's when they will be at the most contagious."
He said he had to rely on parents telling him if their sons were not immunised.
"If your son is immunised then you have nothing to worry about," he told parents in an email today.
"However, if you as a parent have chosen, for whatever reason, not to have your son immunised, then there is a possibility that, due to the nature of this measles outbreak in our city, he will continue to be at risk.
"Additionally, we wish to protect others in our community who are immunity compromised due to other health reasons."