The flight instructor reported that, after demonstrating a simulated emergency landing, he taxied the airplane off the runway and prepared to back taxi for departure. While taxiing, the instructor and student monitored the right-wing tip clearance to the overgrown brush on the right side of the runway, but the right-wing became entangled and the airplane veered to the right. The instructor added left rudder and brake, but the airplane impacted a ditch. Postaccident examination revealed that the right wing struck a tree and sustained substantial damage. The instructor reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The instructor added in a postaccident interview that the airport did not have pavement markings, so he used a pavement crack down the middle of the taxiway as a reference. He added that the entangled brush near the location the wing struck a tree appeared to be more overgrown than other areas along the taxiway.