As an American, I say "principal". The other terms strike me as British. In order to remember the proper spelling of "principal" (as opposed to the homophone "principle"), just remember that your principal is a "prince of a pal" :)

In British English, we would use "head teacher" in primary school and in secondary school. "Headmaster" is now a bit of an old-fashioned term. We don't use the term "principal". Following on from what Phil said - in the UK, the head of the whole university in called the "Vice-Chancellor", similar to the "president" at American universities. The heads of the various departments are also called "deans" at Oxford, Cambridge and Durham, but at most universities, they are called "heads of faculty".

We use principal. They all mean the same thing.

Just teacher but in college it's professor. I'm an italki English native teacher.