Support staff at Keith St School include four permanent teacher aides and three casual teacher aides, two office staff (one of whom takes care of payroll and finances, while the other is also the school's librarian), one caretaker and two cleaners.
Mrs Ireton said the support staff were "essential" to the running of the school.
"Their role is so important."
She said it was "always a struggle" to find the funding to pay for them. Support staff are paid out of schools' operations grant, which means they are competing with the school power bill and building maintenance for funding.
The government has frozen the operations grant, although in last week's budget it announced a 1.3 per cent increase from January next year.
Mrs Ireton said she had no hesitation in signing the open letter, which was organised by the NZEI.
"We are really committed to being fair employers, but in order to do that we have to look closely at the hours we can employ our support staff. We're constantly balancing the needs of the school with the financial needs of the school," she said.
Mrs Ireton said support staff should be paid a living wage.
"At the moment we can't do that."
Chris Dibben from Tawhero School, who also signed the open letter, said his support staff
started on an hourly rate of $17-$18.
"I'd love to make it $20 an hour, but I just can't."
Mr Dibben said job security was a problem for support staff, as most of them don't get paid during the school holidays.
He agreed with Mrs Ireton that they were crucial to the school.
"They play a huge role in learning, and in the school in general. They are really valued in our school."
The other Whanganui principals who signed the open letter were: Jacqui Luxton (St Mary's School), Richard McMillan (Taihape Area School), Phil Walker (Westmere School), Maryann Roberts (Aranui Primary School), Michelle Watson (Mosston School), Sarah McCord (Kaitoke School), Jan Clark (Brunswick School), Stephen Lewis (Hunterville School), Charles Oliver (Whanganui Intermediate School), Jude O'Keefe (Sanson School), Daniel Price (Arahunga School), Ruth McLeay (Faith City School), Terangi Hemi (Te Kura o Ratana) and Jane Corcoran (Waitotara School).