According to the conservation group, these new proposed areas would mean that John Bancroft's modernist gem would become an island in a conservation area sea.
C20 Caseworker Eva Branscombe has complained that the building has purposefully been singled out for exclusion, after planning guidance to demolish the site and redevelop it was mooted earlier this month.
Built between 1965 and 1970 by the Greater London Council's architects department, Pimlico School is marked by a 'tectonic richness uncommon within its architectural genre', according to conservationists.
Pevsner writes: 'The long frontages step forward and backward in a restless, aggressive, exciting way.'
Branscombe said: 'We consider the Pimlico School to be of great architectural merit and indeed feel that it should have been included in the statutory list years ago.
'Its exclusion from the surrounding conservation area will put it in the great risk of insensitive alterations or indeed imminent demolition and redevelopment.'