Principle -
Principle is a noun that means a strongly held belief or a basic fact. It is often used to describe how good a person is. Someone with strong principles is usually seen as a good, moral person.

Examples:
1. He did not steal the money because of his principles.
2. I disagree with her, but I respect her principles.

Principal -
A. Principal can be a noun or an adjective. As a noun, principal means a person in charge of a school.
Examples : One of the jobs of a principal is to visit classrooms.

B. As an adjective, principal means most important.
Examples : Chocolate is the principal ingredient in chocolate chip cookies.