A lady, often but not uniquely, of Catholic education who engages in the time honoured Girls' School pastime of clamjousting. It is the noble martial art of clam-to-clam combat, where one girl grinds her clam against her opponent's. (aka Tribadism)
The collective noun is 'TILT' (From the medieval tournaments consisting of a number of jousts.) "A tilt of clamjousters"
Example: "That dorm has seen so much clamjousting the matresses squelch with fannybatter"
Source: Cutaway, Aug 17, 2004 http://www.urbandictionary.com/define.php?term=clamjouster&defid=797141