Elgin Grabouw Civic Organisation leader John Michels said he had not seen an environmental impact assessment clearance form for the site, which has wetlands on it.
Education MEC Donald Grant recently announced the extra classrooms would be built on the council-owned site and that it had been approved by the developmental planning department.
His department told a newspaper the school would be fenced off from the wetlands.
Michels said the mobile classes, and ultimately a new school, should be built on a sports field close to two predominantly Coloured schools in Grabouw.
"This would be convenient, central and conducive to [racial] integration of the community," he said.
The town was the scene of violent protests last month against overcrowding at Umyezo Wama Apile Combined school.
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