Staff from Tiffin girls' school, in Kingston-on-Thames, will meet ministers this week to plead for cash for a new building.
The fire, which was started by an electrical fault, caused damage costing pound;1 million. Seventeen classrooms are out of action, and the kitchen and all the toilets were destroyed. Hundreds of pieces of coursework were also lost.
Girls are being taught in classes of up to 150 in the school's hall and gym, until temporary accommodation arrives next month.
Insurers are insisting that the 1950s block be rebuilt to previous specifications.
However, the grammar school claims that that would be false economy because, under the Government's Building Schools for the Future proposals, all schools should be rebuilt to 21st-century standards within 15 years.
Pauline Cox, the head, said: "The teaching block was very old-fashioned with no corridors separating classrooms, and these fell 20 per cent short of modern classroom space requirements.
"To have it redesigned and rebuilt to modern standards would cost up to pound;5m, instead of the pound;1m estimated by insurers.
"We do not feel that this is a huge difference in the general scheme of things."
Mrs Cox and officials from Kingston council, met Stephen Twigg, education minister, as The TES was going to press. They were expected to ask him to relax rules which say that low-achieving schools in deprived areas should have priority for rebuilding.
The Building Schools for the Future programme was unveiled last year by the school standards minister, David Miliband.
He said that the initiative would ensure "we have inspirational, well-designed schools to motivate teaching and learning, and up-to-date facilities to enable us to compete as a world-class economy in the 21st century".
A Department for Education and Skills spokesman said this week: "Money from the BSF scheme will not start to be allocated until next year.
"Until then, the school will have to rely on Kingston council for any extra finance to rebuild it."
There are around 20 arson attacks a day on schools, costing pound;100m a year, with a similar number of fires starting accidentally.