The words principal and principle seem to confuse a lot of people. Indeed, Webster’s Collegiate Dictionary, 11th Ed, remarks “Although nearly every handbook and many dictionaries warn against confusing principle and principal, many people still do.” The word principle refers to a set of rules, beliefs, doctrines, or assumptions, e.g. “the principle of disclosure is the heart of U.S. securities regulation.”
The word principal, on the other hand, has two meanings. The first refers to positions of people: “a person who has controlling authority or is in a leading position”. Id. Examples of this are a school principal, the leader in a criminal enterprise, the person who engages an agent to work on this behalf, the lead actor/actress in a play, the person primarily obligated on a debt. This use is easily remembered by something I was taught as a kid: a school principal can be your pal but a principle cannot. So where you want to use the word relating to a position of a person, think of your pal.
The second meaning is kind of related to the first: “a matter or thing of primary importance”. Id. Examples of this are the principal (base) amount of debt (as distinguished from interest), the principal (main) ingredient in a recipe, the principal (corpus) of an estate or trust, etc.
In short, principle only refers to laws, ideas, beliefs, doctrines, etc. If you want the word referring to the other meanings, you want principal.