Best to include some challenging and some easy games/drills. They get easily discouraged when they fail. Keep to basic skills such as early backswing (racket back before the ball bounces), follow through (tell them to 'bite the marshmallow' inside elbow to finish the forehand), contact point control (let them bounce balls and catch them in ball tins to practise the eye). A group of 12 is very big and difficult for one coach alone. Let 6 play an easy but fun game one 1 area of the court, while you focus on the technique of the other 6 with drills. Hope it helps