The Office for Standards in Education reported that some schools had stopped providing special support for more advanced bilingual learners to give priority to the growing number of new arrivals who could not speak the language.
However, inspectors found that bilingual pupils who spoke English fluently could still struggle with their studies, even if they passed GCSE English.
One key stage 4 teacher told inspectors: "The mismatch between oral communication and writing ability is quite stark for some pupils."
Ofsted analysed test scores of pupils who spoke English as an additional language (EAL) and found that Somali, Kurdish and Turkish speakers performed considerably worse than others.
Ofsted also published separate research exploring the writing difficulties of EAL pupils, who make up a tenth of students in English schools.
It found that bilingual pupils tended to make the same kinds of grammar and spelling mistakes as native English speakers with below average GCSE English results.
The report also noted that Muslim pupils often had problems writing about their religion objectively.
A study of 36 religious education essays by Muslim students found that a quarter "used explanations that would be appropriate from a 'believer'
stance, but less appropriate from a subject discipline or 'pupil taking exam' stance".
"Writing in EAL at key stage 4 and post-16" and "More advanced learners of EAL in secondary schools and colleges" are at www.ofsted.gov.uk