However, Whanganui Girls' College principal Tania King said the ministry had not confirmed with her that the building would be demolished in December. She thought that was unlikely to happen, since the work had not yet been put out for tender.
"So we're looking at six weeks before this demolition is due to start. I think it's more likely that it will start at the beginning of the school year next year," Ms King said.
She said A block, which contains 10 classrooms, hasn't been used for the past two years.
It dates from the 1950s, when the roll was at its peak of 1200. Currently the school roll is at 360, and growing.
Assistant principal Prabh Mokha said the idea for a new wharenui came from a parent focus group.
"We have a whare at the back of the school, and it's really just a tin shed. We use it for kapa haka practice.
"We've had it in our heads that we wanted a wharenui that was more front and centre, but we never had the space for it."
Mr Mokha said the wharenui would be nearly 100sq m, and would be used for cultural events such as pōwhiri, functions, performing arts and teaching. It would have a kitchen and a bathroom.
The plans have been drawn up by architects BSM, in consultation with local kaumatua John Maihi.