Gammer Brevis is a reasonable witch when provoked (which in turn provokes seething fury in Granny Weatherwax), who teaches over the mountain from Lancre. She appears as a coven member in Witches Abroad, discussing who should take over Desiderata Hollow's cottage with Granny Weatherwax, Nanny Ogg and Old Mother Dismass. She was the last person to see Mabel Peavey before her mountain-climbing accident.
A very similar witch is mentioned in The New Discworld Companion as Gammer Beavis. A witch of this name also appears in the short story The Sea and Little Fishes; these may be the same character as Beavis is described by Nanny Ogg as 'a bit too educated' which would fit with her being a teacher. She does her own shoe repairs and takes snuff and thus, in the opinion of Nanny Ogg, is "all right".