She estimated it would have cost around $400 if she had bought his entire uniform new, compared to the $20 she spent.
Conor had grown about a head in the last year.
"Even if you are well off they grow so quickly," Mrs Mitchell said. "What is the point getting a new uniform every year?"
While Conor did not need to bring a laptop or iPad to school at this stage, Mrs Mitchell said she would not be surprised it it became necessary at some point.
"We're not doing that because it is not compulsory which I'm grateful of because some schools are compulsory."
Mangere Budgeting Service chief executive Darryl Evans said school costs were getting higher but people on low incomes were rarely able to save for them throughout the year because rents and other living costs were rising too.
"I know of a few families who won't send their kids to school in the first few weeks because they haven't been able to meet the costs," he said.
A new one-for-all uniform available at cheaper retailers was being embraced by some schools as a way of keeping costs down - but others were demanding kids turn up with more expensive uniforms and equipment than ever before.
Family Budgeting Services chief executive Raewyn Fox said the cost of back-to-school stationery - once the biggest expense for parents - was now meagre compared to the uniforms and electronic devices some schools required.
"I have a sense back-to-school costs are improving overall but we still get families every year that are struggling and now there's new pressures of having tablets and so on that didn't used to exist," she said.
Two other areas of difficulty for many parents are school and special activity fees, she said.
"Some families steer their children towards subjects with low activity fees so it's not so much of a burden on them but then that means a number of other issues for the kids."
-All Northland schools start between Tuesday, at the earliest, and Thursday, February 5, at the latest.
Retail cost of school uniforms online:
*Blazer: $25 - $240
*Jersey: $58 - $108
*Trousers: $25 - $70
*Shorts: $4 - $65
*Kilt/skirt: $30 - $215
*Blouse/shirt: $3 - $45
*Tie: $1 - $30
*Prices at the lower end of the range are second-hand, and those at the top end of the range are brand new.
Five tips to save costs
1 Trade Me is a popular avenue for parents wanting to buy and sell second-hand uniforms. Yesterday, there were 52 listings for school uniforms in Northland.
2 Local supermarket community noticeboards usually have notices of people selling second-hand uniforms.
3 Most schools and Parent Teacher Associations sell second-hand uniforms - contact your school for details.
4 Second-hand shops - many second-hand shops have uniform sections.
5 Talk to family and friends who had children at your school - many parents will still have their children's old uniforms.