The Witches League is an organisation for pureblood girls. It is known to be opposed to any influence of muggles in wizarding culture and teaches its members that view. Members learn skills deemed fitting for a respectable witch. What those skills are exactly is not clear, but learning to defend yourself against dangerous muggles is defeinitely part of it.
Their counterpart for boys is called the Trailblazers
The Witches League has chapters in different places around the British Isles, led by Group Leaders. If there is a central chapter, and who leads it, is not known.