Do you have to go to medical school or law school or graduate school?

No, you don't. You can make a living doing things that do not require a degree. But if you do want to become a medical doctor, you must graduate from medical school (and pass an internship). A lawyer must graduate from law school (and pass the bar). Graduate school (for the advanced degrees like the Master's or Doctorate) is a great place to hone to a bleeding edge your chemistry, math, language or historical knowledge to either work deeply in your field or to teach it at the collegiate level.

No, you don't have to do any of that. But you might want to think about at least starting down that road to see what's there, and what's inside you.