Union Station - Group Critique Busted!

When I entered the great hall in Chicago's Union Station, I saw about 10 art students sketching the interior. I walked around for 15 or 20 minutes taking photos. When I was leaving, I noticed they had put their sketch pads on the floor and had gathered in a circle. I assume this was a group critique. I thought I could sneak up on them, raise my camera with fisheye lens above my head and get a quick shot. You can see I got busted.