A flibbertigibbet is a very silly chatterbox. If your teacher calls you a flibbertigibbet, she clearly doesn’t think you’re a serious or scholarly person.
The noun flibbertigibbet is a colorful — if slightly old fashioned — word for a ridiculous busybody, someone who talks and gossips a lot but doesn’t have much of any substance to say. It’s most often used to talk about a frivolous young woman. Flibbertigibbet is a Middle English invention, a word that sounds like meaningless chatter, and it’s still used fairly often in Yorkshire, England.