Principal is the person or the main thing. Principle is a rule or belief. Spell-check will not catch the swap. The principal of the school. The principal reason. A principle of fairness. Diction as a set is in word choice.
Person, main, or rule
If you can swap in head of school or main, use principal. If you can swap in rule or belief, use principle. The pal at the end of principal is a cheap but useful check.
Advise versus advice is the same kind of look-alike, in advise versus advice. Affect versus effect is in affect versus effect. Grammar overall is in ACT grammar rules.
The bottom line
Person or main: principal. Rule or belief: principle.
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