"She wears it with a smile," Thornton says. "I think when you're the youngest you want to have some new things, but she knew that some parts of her uniform were going to be passed down from her sister."
The Riverhead family still find the cost of kitting out three kids - about $500 per child for school uniform and sports kit - to be "just ridiculous".
A uniform price list on the Westlake Girls' school website shows it would cost between $750 and $1300 to kit out a girl in the school's complete uniform and PE kit.
And while schools like Westlake Girls are particularly expensive, the problem is nationwide.
Thornton is a nurse manager for a Well Child service provider in West Auckland and has heard many other parents complain.
"I've heard of $45 for a polo shirt," she says. "I can't understand why uniforms are so expensive, given the cost to mass produce them. They're not even the best quality. Someone's got to be making a [lot] of money."
She wants more competition in the school uniform market, rather than the commonplace exclusive uniform agreements.
And that is a sentiment shared by Westlake Girls' new principal, Roz Mexted.
For now, The Uniform Group supplies uniforms to the school's shop, which then sells them with a mark-up. But that system is under review, Mexted says. "We're looking at what's best for parents. I don't believe schools should be making money out of uniforms or stationery. We need to make sure what we're doing is fair."
Any new deal between the school and a uniform supplier would consider the benefits to both the school and parents. "It's a costly thing to outfit students in new uniforms."
A supplier who would help lower-income parents would be looked upon favourably, she said.
Justine Thornton welcomed the proposed change, saying families were being forced to make sacrifices and that was frustrating.
"I think there's other things you could be putting money towards - like more tuition, extra-curricular activities, even just taking your kids out for an icecream, anything you can do to show your kids the positives of being a parent."