The heritage group has commissioned Robert Loader Architects to produce two alternatives to the proposed Future Systems scheme, which plans to knock down the school as part of a wider regeneration scheme in the Elephant and Castle area.
The first of the C20's proposals (below) offers a New Kent Business Quarter and a Geoffrey Chaucer Academy, which keeps the listed school.
According to the society, the scheme redevelops the 'under-used sites along the New Kent Road, and reappropriates the existing listed buildings for continued educational use, as well as providing new facilities'.
The second scheme (bottom) is strictly a business development, called the New Kent Business Quarter Strategy. The scheme retains the New Kent Road proposals from the first scheme, but completely transforms the school buildings into small business units and workshops.
A statement from the society said: 'C20 sees either scheme as a viable solution in response to the needs of both the school and the community.
'Schools are a key battleground for the society, with more and more, particularly post-war examples, being earmarked for demolition or change. If this listed example should be demolished, without a rigorous and creative examination of the possibilities, it would set a very worrying precedent.'