During the third practice soft field landing of the instructional flight, the student pilot flared the airplane and it entered a "full stall" at 5 to 7 feet above the grass runway. The airplane touched down and the flight instructor pulled back on the yoke in an attempt to recover the landing. The nose landing gear then struck the terrain and separated. Examination of the airplane revealed the firewall and the top of the left wing were wrinkled resulting in substantial damage to the airplane. The flight instructor and student pilot were not injured. The flight instructor did not report any mechanical problems with the airplane.