The head of a high school is "principle."
What is the head of a kindergarten?
The title for the head of a kindergarten would depend on many variables, including the country. In the US, a kindergarten is usually part of a public or private school, therefore, its head would be the head of the whole school, i.e. the principal (public school) or the principal, director or headmaster (private school.) If it is a free-standing kindergarten, (I'm not sure such a thing exists in the US) we'd probably call the head the director.