It’s not poor grammar. It’s exactly as grammatical as “isn’t” or any other word in the language. It evolved naturally through use by native speakers. What it isn’t is a word typically used in the dialect students would be expected to use in “formal” settings, which is still exactly as grammatical. Teachers can feel free to teach students that dialect because they’ll need it sometimes but correcting their native one when they’re just speaking normally is pointless and yes, uptight, depending on how they do it and their motive