The best teacher I EVER had was a history teacher at my private highschool. He had a PhD in history and absolutlely no education or teaching credentials. I would recommend finding a private high school at which to teach. The pay will be less as will the benefits, but you won't have the paperwork nightmare and BS schoolboard, govt. oversight on what you are doing. You also more likely to have students wanting to learn.

Depending on you, your abilities, personality, etc. You may fit in nicely in this environment without any extra credentialing. That being said, they will get rid of you or ask you to get more formal training, if you stink. Just my 2 cents, after watching my wife struggle in the public sector (doing well I might add as teacher of the year x2) but moving to a private school because she couldn't take the BS anymore.