Located in the United States (presumably in Salem, Massachusetts), this is a social organization for adult witches. Members attended the Quidditch World Cup in 1994 (GF7)
@jgrahamhutch The Salem Witches' Institute isn't a school, but a joke on the Women's Institute in the UK.
The Women's Institute in the UK is a volunteer organization for women which focuses on social activities, crafts, cooking, and good health (Wikipedia).
Salem, Massachusetts, is infamous for the witch trials which took place there in 1692 (http://en.wikipedia.org/wiki/Salem,_Massachusetts).