Rolleston College agreed in December to new classrooms, a tech block and gym at its senior campus west of Christchurch.
But tumuaki Rachel Skelton said that was now being cut back, even though officials told her the school was the number one priority in a new school building pipeline.
"I'm horrified - if that is what the top priority school is given, I'm really concerned about what other schools are being offered," she said on Monday.
RNZ has asked the Ministry of Education for details about the pipeline.
The ministry is reviewing 350 new school builds and renovations after the government slammed it for under-delivering amid cost over-runs.
Skelton said the ministry told her last Thursday this review would not impact her college's big build.
However, she also learned it had "chosen to strip back, redesign, and delay the new campus", with a two-year delay till 2026 to even start a build meant to start in mid-2024, and drastic plan changes, she said.
Instead of classrooms, the new plan was for "makeshift portacom buildings indefinitely", with no purpose-built fit-outs, a school statement said.
The science and tech block would undergo minimal alterations, but a gym redesign eliminated all classroom space, and space for nurses and counsellors and a staffroom.
"The presented plan means the campus cannot function as a school and is completely inappropriate."
The ministry had costed the stripped-back plan at $52 million - and told her they would need to find further savings on top of that, Skelton said.
The school was not consulted on the revised plan - it was presented as a done deal, she said.
"We're just absolutely baffled, we're devastated."
The plan agreed in December was to cater for years 11-13 in a senior campus 2.5km from the main campus, envisaging roll growth to 2100 senior students, in the fast-growing Selwyn district, which was about to add a tenth primary as a feeder for Rolleston College.
The site works were already done, and billboards had gone up promoting what had been a three-stage build.
"They said categorically there was no appetite or commitment to continuing with stages two and three."
The school is asking senior government ministers to visit to hear their plea.
In the meantime, Auckland Girls' Grammar School had believed it was poised to get a new gym, but now worries it will not.
Its gym has been described as "sliding" down a slope since incurring storm damage.
"We have a cohort of Year 13 students who have never accessed their PE/Health curriculum through a gymnasium facility as the gym has been closed because it has been unsafe all of their time," principal Ngaire Ashmore said.
"We thought we were just about over the line with with progress and money to fund the remediation process but the funds have come up short.
"We are hoping (fingers crossed) that the MOE will approve the extra funding needed to reinstate the gymnasium."
