The operator reported that the purpose of the flight was to provide initial off-airport training to a pilot in a tailwheel-equipped airplane. The pilot receiving instruction was seated in the front seat, and the pilot providing instruction was seated in the back seat. After completing a series of stop-and-go landings at various off-airport sites, the pair landed at a 600 ft gravel-covered site. After a brief stay at the site, the pair elected to continue practicing stop-and-go landings. During the second landing at the site, as the pilot receiving instruction was manipulating the controls, the airplane bounced slightly at touchdown, then it touched down again in a three-point landing. As the landing rollout continued, about half-way down the site, the airplane decelerated quickly, and the tail began to rise. In an attempt to lower the tail of the airplane, the pilot providing instruction applied full aft elevator, and tried to instruct the pilot receiving instruction to correct the excessive braking action. The airplane subsequently nosed over, sustaining substantial damage to the rudder, and wing struts. The operator stated that there were no pre-accident mechanical failures or malfunctions with the airplane that would have precluded normal operation.