The Oxford English Dictionary says "gramarye" first meant grammar or learning in general. The secondary meaning of necromancy though known in the Middle Ages, apparently fell out of use and, according to Oxford, was consciously "revived" by Sir Walter Scott in 1805.

So "gramarye" in the song could mean either, but if the text originated in the 17th or 18th century (the usual case), the odds are that "grammar" is all that's intended - with an extra syllable for the meter and rhyme.

Of course, one remains free to imagine whatever makes for a better story....