Note: The differences between principal and principle are discussed in depth in the Common Grammatical Mistakes section of Grammar.com. Click here for that discussion.
The principal (who should be your pal) is the head of a school. As a noun, principal also means “money” or “the balance on your mortgage.”
As an adjective, principal means “main.”
The word principle acts only as a noun. It means “rule” or “moral tenet.”
Use this trick:
Principle ends in -le. So does rule.