expressions – Which one is correct? “A teacher of English” or “An English teacher”? – English Language & Usage Stack Exchange

Both are grammatically correct, and which one to use will depend on the context.

teacher of English: This makes an explicit reference to the subject. It doesn’t matter if the teacher is from England or not, it just cares about the fact that the teacher teaches English.

Examples where teacher of English is used:
http://www.ppef.cz/best-teacher-of-english-award/
http://www.teacher-of-english.com/
http://inspiringenglishteacher.sg/ (its funny to see here how the award is for Teachers of English but the URL is a contraction of “Inspiring English Teacher”)

English teacher: This can be ambiguous depending on the context because English can relate to nationality or to the subject English. If such an ambiguity is not possible in your context, English teacher is shorter, easier to pronounce and the natural choice of words. It would even sound a little awkward if you, at your own school, should refer to your English teacher as your teacher of English.

Examples where English teacher is used:
http://en.wikipedia.org/wiki/The_English_Teacher
http://www.imdb.com/title/tt2055765/
http://crofsblogs.typepad.com/english/