That mnenomic hasn't been taught in schools for decades, but perhaps it should be--not to help people spell the title of the person who's the head of a school but to help them choose the correct spelling in every other situation calling for a noun or an adjective with the meaning "head," "leading," or "most important."
In recent days, I've encountered principle author, principle stakeholder, principle access road, and principle engineer (that one could actually work--an engineer of principles?) not in the Register-Star but in the work of some writers who should know better. (Can you guess what I've been reading lately?)
Principal can be a noun (principal of a company) or an adjective (principal dancer). Principle, on the other hand, can only be a noun. (Of course, principled is an adjective, but I've never known anyone to spell it principaled.)
That's more than enough pedantry for now--especially from someone whose recent late-night post contained one amazing blooper. It's a matter of principle.