Principles/Principals

There are at least five uses of the word principles(pals) but two different ways to spell it.

Principle: Guideline or rule

Exmaple: The basic principle of investing is to buy low and sell high. 

Principle: Moral guideline or conduct

Exmaple: It's against my principles to lie.

Principal: Main or major

Example: The principal reason I called this meeting today.

Principal: The head of a school

Principal: Capital sum of money, as distinguished from interest.