No, it's not a silly question, because in fact you could say either. 'A history teacher' is more usual, however. 'A teacher of history' is correct, but a little formal and old-fashioned sounding.

Not a silly question at all. A native speaker would say "She's a history teacher." If you say "She's a teacher of history", some people may misinterpret this as "the teacher having a history", though some native speakers may say "She's a teacher of math or She's a teacher of science" -- it just sounds a little awkward. :-)

"History teacher" is the more natural way to say it. "Teacher of history" is perfectly correct, perfectly clear, and is not awkward. I would be more likely to "teacher of" in these cases: a) When the subject is not a standard school subject; "She's a history teacher," but ... "She's a teacher of paleontology" "She's a teacher of forensic science" b) When the name of the subject is a long phrase, or when begins with the word "the:" "She's a teacher of the history of science" "She's a teacher of applied science, engineering, and technology" c) When "X teacher" form is ambiguous because X contains an adjective that could apply to the teacher herself: "She's a teacher of Chinese history," not "she's a Chinese history teacher." If I just wanted to explain what she does, I might say "She teaches X" rather than "She's a teacher of X". "She's a teacher of X" suggests not only that she teaches X, but that her formal job title at the school is "Teacher of X."

Thank you all! you're really nice. Thanks for your help :)

1) The only silly question is the question you don't ask!