The bilingual science lessons for Turkish pupils at White Hart Lane school in Tottenham, north London will be phased out next term.
Joan McVittie, the school's new head, is also concerned the scheme might deter Turkish pupils from mixing with others.
"The children should be proud of their heritage," she said. "But these are young people growing up in London. We need to prepare them for work and life in London, so when they are in school they should commuicate in English."
The bilingual teaching scheme was introduced in 2001 by David Daniels, then headteacher, who said it would be a boost to Turkish students and would be extended to Somali pupils. Later, Stephen Twigg, then education minister, told MPs it was "very much the kind of good practice we want to promote".
More than 70 per cent of White Hart Lane's pupils have English as a second language and 65 languages are spoken by students. Around 30 of its 400-odd Turkish speaking pupils have received bilingual lessons each year in preparation for science GCSE.
Mrs McVittie said she was disappointed by the school's science results: only 23 per cent of pupils got a C or better at GCSE - and the figures were no better for those taught in Turkish. She said that forcing pupils to speak English in the playground was unrealistic, but that she was setting up a buddy system so pupils with the same mother tongue could help less fluent friends to speak English.
Mrs McVittie said she was confident other reforms at the school i would help to improve results.
Last week the National Assessment Agency said primary children not fluent in English could have translators for maths and science tests.