It's actually a case of the British intrusive R, they are schools for grandmas

What’s use if grammar

common misconception, those are schools that were founded by educational icon Kelsey "Frasier" Grammer

Grammar is just the angloid spelling

Innit

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Wait till you hear what they teach in finishing school.

Serious answer: it's derived from γραμματικὴ τέχνη (grammatikḕ téchnē), or "art of letters", so originally referred to the whole of written language (and sometimes spoken parts too) rather than just the word order. Grammar schools were for learning how to read and write Latin and Greek.