When I was in junior high, we were taught that the way to differentiate these homonyms, or sets of words that sound alike but mean something different, was to think that the principal was our pal. That way we would remember that principal refers to a person whereas principle refers to an idea.
Thomas Jefferson stood on principle.
A high school principal has a tough job.
This one is pretty simple once you remember the rule. I recognize some teens may gag on the pal reference, but if it improves your grade, what the hell?
Sigrid Macdonald is a manuscript editor and the author of Be Your Own Editor. Find her at http://sigridmacdonald.