1. Principal (as in "head of a school") - This is spelled principal and is pronounced with the stress on the first syllable.
2. Principal (as in "main" or "most important") - This is spelled principle and is pronounced with the stress on the second syllable.
It's a common mistake to mix these up, so remember:
* Principal (with one "l") is the person in charge.
* Principle (with two "l"s) is a rule or belief.