"Principal" instead of "principle": spelling error or grammatical error
Solution 1:
It's neither of them really. It's a lexical error, meaning an error in choice of words. Grammatical errors are really just violations of the patterns of our language. Spelling errors are unintentional, so if you are really convinced you're using the right word, it doesn't count (thus they can be hard to differentiate from lexical errors in finished works, but that's what proofreaders and editors are for.)