Oh, well, good news then, if you actually examine the curriculum requirement for being a history teacher and compare it to getting a degree in straight-up history, you'll see that the teacher is not required to take anywhere near as much, and the teacher will not take the really hard history courses that grade in terms of reams of essay writing.

Some teachers take the initiative to double-major of their own free will (respect!), but it is certainly not required. When I was in school I casually assumed that my teachers knew a great deal more about the subjects than what they were teaching; when I got to University and had the opportunity to look at their curricula requirements I learned that was not true, at least not in general.