St Marylebone CofE comprehensive in central London had to turn to the scheme, sponsored by Government and business, after losing 13 of its 70 teachers.
John Hunter, deputy head, said an "appalling" settlement had left the school short of pound;300,000 on a pound;3.7 million budget. This had forced seven redundancies, three of which were compulsory.
Six other experienced staff left the Westminster girls' comprehensive for other jobs: some were prompted to move on by the school's financial difficulties.
One post was filled by a qualified teacher, but five were left vacant which forced the school to cut four A-level courses.
The only way gaps could be plugged was by taking on seven unqualified trainees through Teach First.
The scheme, set up earlier this year, has placed nearly 200 high-flying graduates in London secondary schools to help relieve staff shortages. The graduates have no teaching experience and just six weeks' training. After two years in schools they get special consideration for jobs with the scheme's blue-chip business backers.
Mr Hunter said his school would have preferred to take on just one or two Teach First trainees.
But at pound;17,000, pound;10,000 less than the salary of a teacher with two years' experience, they meant the school could avoid cutting another 2.5 teaching posts.
"They are all keen and enthusiastic and motivated but they have not taught before," said Mr Hunter.
"However, if it wasn't for Teach First we would have even fewer teachers."
A Teach First spokeswoman said she expected the scheme to expand due to the demand from cash-strapped schools.
Mr Hunter said that to attract experienced staff the school would have to offer extra financial incentives.
Its success - 93 per cent of pupils gained five good GCSEs last year - had been built on experienced staff, he said. "If this keeps happening it will affect the teaching in a big way. It will be very serious for the school."