I teach 9th and 12th grade and I've had my class in groups ("islands" of 4 desks) since September. It works really really well in my 9th grade classes because I don't do a lot of straight lecturing and they do a ton of group work. However, it's a nightmare in my 12th grade classes because 1/2 of them have their backs to me and I feel like the group setting just creates an atmosphere that's too sociable/laid back for the seniors. I'm thinking of the good ol' horseshoe. Those of you that teach high school, what's your favorite?