These two terms are often confused, but the fact is they have no meanings in common. “Principal” means “first, primary, or main”:

The principal feldspar is orthoclase.

“Principle” implies an abstraction, and it means “a doctrine,” “a truth”:

MAN01 teaches basic management principles.

“Principal,” of course, also denotes the head of a school, and in some fields, “principal” has specialized meaning: in law, a principal empowers another to act as his or her representative; in finance, the principal is the capital of a financial holding as distinguished from the interest.