There are nouns
A principal is head of a school (most commonly in America).
Mr Fitzgibbons was the principal at the time, and he never did like me.
A principle is a moral rule, or a general truth.
I understand the principles of chemistry.
I object on principle.
And there are adjectives
Principal describes something that is foremost in importance and value.
Our principal publication is the newspaper.
A principled person or action is one guided by moral rules.
He made a principled objection to eating horsemeat, on the grounds that they carry us and our burdens so mankind should ask no more of them.
You can trust the vicar to judge the pumpkin contest fairly. He's a principled man.
Getting it right
A principled person is led by his principles; and if you suck up, the principal (a person foremost in importance and value) could be your pal.
