Rolleston College has launched a range of actions, including a petition and community meeting, protesting a Ministry of Education decision to slash the design for its new campus without consultation.
Principal of the Canterbury school, Rachel Skelton, said the changes are "substantial".
"Instead of classrooms, they have proposed to use makeshift Portacom buildings indefinitely, as learning spaces with no purpose-built fit-outs. While the science and technology block will undergo minimal alterations, the gymnasium's redesign eliminates all classroom spaces, hauora services for nurses and counsellors, and leaves staff without essential facilities," she said.
The school was not consulted on the revised plan - it was presented as a done deal, Skelton said.
The ministry defended the scaled-back plans for the senior school campus, saying it reflected what the school would need when it opens.
It was reviewing 350 new school builds and renovations after the government slammed it for under-delivering amid cost over-runs.
Population growth in the area was high and "additional capacity... further stages of development will continue to grow the campus over time as the college roll grows", it said.
Selwyn was one of the country's fastest growing regions.
Skelton said the school's roll was predicted to grow to 3000 by the early 2030s.
"That's approximately 1200 more students we need to cater for in six years. Our current campus is already at capacity with a roll of 1800 students and this will further strain resources, impact learning environments, and place added pressure on students, parents, and our dedicated teaching staff."
A petition had been set up against the new plan, which had gained 1200 signatures in a day, and a video message from Rolleston College's head students to Education Minister Erica Stanford had been posted to its website.
The college was also encouraging the community to write letters to the minister and was hosting a community meeting on the revised plans next week.
"The senior leadership team invite you to a meeting at Rolleston College to share your concerns, experiences, and aspirations for the new campus. Here we can provide updates an answer all of your questions in person," the meeting invitation said.
Ministry representatives and Selwyn's mayor would be invited to it too, the school said.
It would be held on 13 March.