Auckland Grammar School says it will not change its short-hair rule for a boy who wears his hair shoulder-length in memory of his grandfather.
The boy, 11-year-old James, was born one day after the sudden death of his grandfather Paul Hunt, a musician who was expelled from his own school for long hair and kept his hair long until he died aged 55.
James' mother Heidi Hunt said James had always had long hair too.
"It's who he is," she said.
She plans to move out of the small rented Epsom flat where she and James live, and has written to the school asking it to change its rule against long hair before she decides where to go - and before James goes to high school in 2019.
"If he can't go to Grammar with his hair, there's no point staying in the Grammar zone," she said.
But the school bans any hair long enough to touch a boy's collar, and headmaster Tim O'Connor said the rule would not change.
"We articulate our rules very, very clearly, and they are fair and reasonable, and it's up to parents whether or not they would like to send their boys to Grammar with all of that information in mind," he said.
"I'm sure there are some people who have issues with other aspects of how we educate young men.
"My advice to them would be to choose another school for educating your son."
James, a year 7 student at Auckland Normal Intermediate, is a top cricketer currently ranked 10th on CricketHQ's recreational cricket MVP leaderboard for Most Valuable Players nationally, despite his young age. He is keen to go to Auckland Grammar with his friends.
"I want to go to Auckland Grammar because they have good education there and they are really excited about sports, especially cricket," he said.
But he is not willing to cut his hair - "because my granddad had long hair and he got expelled for having long hair and he died the day before I was born".
His mother said James had lived in the Grammar zone all his life and was entitled to go to the school.
She said she was not forcing James to keep his hair long.
"If James wants to cut his hair, I'd let him. But I don't want him to have to," she said.
Her letter to O'Connor cited a 2014 High Court decision that St John's College in Hastings suspended 16-year-old Lucan Battison unlawfully because its rule on hair length was vague and uncertain.
But Youthlaw solicitor Jen Walsh said the judge in the Battison case did not rule on whether clearly stated school hair rules breached the Bill of Rights.
"This issue has never really been challenged in court," she said.
She said it could be argued that schools breached the right to religious freedom if they did not make exceptions to their rules for students who did not cut their hair for religious reasons.
"In this case I'm struggling to pin it on any particular element of discrimination, but obviously it does seem quite harsh," she said.
O'Connor said the Grammar board of trustees reviewed its hair rule after the Battison case and decided that it was legal. It is published on the school's website.
"I'm not going to debate our school rules," he said. "We make decisions that we think are right and we will continue to do that."