grammar – “I am a teacher of English.” or “I am a teacher in English.” Which one is correct? – English Language & Usage Stack Exchange

A teacher of English teaches the English language as a subject. A teacher in English uses the English language as the medium in which to teach their subject. For instance there might be a physics teacher who teaches in English and students who did not understand English would not learn much physics ftom them.

To some extent all teachers of English do teach in English becsuse they have to but someone who taught English as a foreign language in English would not use, maybe would not know, the native language(s) of their student(s). English classes for immigrants often have to be taught in English because the students have different native languages. A teacher of English in a French school, however, would normally teach partly in French.