The flight instructor reported that the pilot receiving instruction performed a wheel landing on a runway with a crosswind. The landing was normal until the tail came down. The nose of the airplane went to the left and then back to the right. The flight instructor applied full left rudder, but the airplane continued to the right. The pilot receiving instruction applied full left brake; however, the airplane continued to the right, exited the runway, and came to rest in a drainage ditch. The left main landing gear folded underneath the airplane and punctured the airplane floor which resulted in substantial damage to the lower fuselage. Both the pilot receiving instruction and the flight instructor reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. At the time of the accident, the pilot was landing the airplane on runway 17. The flight instructor reported that the wind was out of the south southwest at 10 kts.