This confusion is also something I see frequently while proofreading. In fact, I have just completed proofreading an article where it was spelled correctly only once out of twelve incidences of the word over two pages!
Principal is a noun meaning ‘chief’ or ‘most important’ person, such as in ‘the principal [head teacher] of the school’ or as an adjective, as in ‘the principal export is gold’.
Principle is a noun meaning ‘general truth’, ‘law’, or ‘rule’, such as in ‘he refused to abandon his ethical principles’.
Stephen York is a freelance proofreader with over 25 years' experience in book and journal publishing offering proofreading services to publishers, businesses, organizations, educational institutions, academics, students, and authors. He regularly proofreads in digital format a wide variety of media in an extensive range of specialist subject areas, including business, finance, economics, education, marketing, and real estate.