Durand Academy in Stedham due to open in October
A school described as England's first free state boarding school is due to open to pupils in October.
The Durand Academy applied to convert Grade II-listed buildings in Stedham, West Sussex into a 375-pupil school, but permission was denied in 2013.
The academy, which runs a primary in Stockwell, south London, is appealing against the planning decision.
A spokesman said they were committed to the scheme, for children from disadvantaged backgrounds.
The South Downs National Park Authority, which turned down the planning application in December, said the school would be too big and its impact on the landscape would be "inappropriate".
The 20-acre site was formerly St Cuthman's School, which had 100 pupils, but the building is now empty.
The school said 48 pupils would spend their first few weeks in Stockwell while renovations are completed, before moving to board in the village next month.
Their specific start date has not yet been finalised.
The Department for Education pledged £17.3m for the school with the Durand Education Trust, which owns the site, providing a further £5m.
But many local residents are opposed to the plan.
Adrian Hearle, a parish councillor who lives opposite the school, said: "The problem is the size and the scale of the new building."
He added that up to six coaches would be travelling down narrow lanes when the children arrive on Mondays and leave on Thursdays or Fridays.
In a statement, the school said it was looking forward its vision becoming a reality.
"This ground-breaking project will deliver both education and conservation in the heart of the South Downs National Park, and transform opportunities for children from some of the country's most disadvantaged backgrounds," it said.
"The children deserve to be given this opportunity to learn and flourish in this unique educational environment, the likes of which are usually the preserve only of the very wealthy."