I teach history and love it. There are so many dialogues that lead to meaningful learning. In terms of private school, that might work well. There is usually a lot of freedom regarding your teaching methods and curriculum. I'll mention some pros and cons of high school and middle school:
Teaching middle school
Pros - kids are energetic, fun, just beginning to create convictions about life, they have been raised around technology, which makes integration easier
Cons - they're junior high kids, which makes classroom management tough
Teaching high school
Pros - more mature, able to think well about life, sense of humor is more developed, better work
Cons - they can be apathetic or think they know everything
"The unexamined life is not worth living." - Socrates