Principal is either a noun or an adjective, but both uses have very similar meaning.
Principle is simply a noun.
Principal (noun): The most important person in an organisation.
e.g. “All of the students love John, he’s a really good principal.”
Principal (adjective): The first in importance, rank, value, etc.
e.g. “The driver fell asleep at the wheel, this was the principal reason for the car crash.”
Principle (noun): A standard or a rule.
e.g. “As a matter of principle, I don’t accept gifts from men.”
To remember the difference between the two nouns, here’s a quick tip:
Always remember, the principal or your school is your pal!
Female Esl Coach 