Ms Igrisan said it was time to "stop the bullying", adding: "It is a standover tactic which is costing parents."
Last year, Manurewa High canvassed several companies before choosing New Zealand Uniforms as exclusive supplier.
As part of the deal, NZ Uniforms agreed to provide a sponsorship programme for disadvantaged children at the college.
However, The Uniform Shoppe continued to sell the school's green and black uniform, as it has done since 1960.
This week, Ms Igrisan received a letter from a legal representative of the school board warning her it would apply for a High Court injunction if she did not stop.
Ms Igrisan said schools were lured by promises of sweet deals by companies wanting the exclusive supply "regardless of the effect on their school families".
Her company compared the cost of seven senior boys' uniform garments - the total at The Uniform Shoppe was $32.30 cheaper than the designated supplier.
But Manurewa High principal Salvatore Gargiulo said incentives did not enter their decision about which supplier they chose - they wanted affordable, good-quality uniforms.
"Janet was so far off the price of the other competitors she didn't even make the shortlist," he said.
"For us, it was the best price and deal that was the top of the list."
But Glennis Godfrey, 66, who has a granddaughter at the school, supports Ms Igrisan and said it was "jolly silly" that there was no competition.
"It's just unfair really ... Some of the parents can't even afford the uniforms."
The hems on Mrs Godfrey's granddaughter's skirts - bought at the start of the year - had already fallen down and the shirts were wearing thin.
The Commerce Commission said the school had not breached any of its guidelines regarding contracts with uniform suppliers.
The guidelines were drawn up in December 2008 after a number of complaints.
At that time, Ms Igrisan battled Rosehill College in Papakura when it settled on an exclusive supplier, locking her out of the market.
Guidelines
* Recommend schools use their "bargaining power" with manufacturers or suppliers to negotiate competitive deals.
* In exchange for exclusive rights, suppliers may provide rebates or gifts.
* School boards should consider the needs of parents.
* When suppliers are being selected for an exclusive arrangement, school boards should ensure the selection process is transparent.
