The flight instructor reported that, with the student pilot on the controls for the landing, the airplane floated and eventually touched down about "the first third of the runway." The airplane bounced, and the instructor announced that he was assuming control of the airplane. The instructor simultaneously attempted to apply full throttle by pushing the student's hand, which was still on the throttle and prevented him from achieving full power as he attempted to gain altitude without stalling the airplane. The airplane subsequently struck a fence near the departure end of the runway, so the instructor reduced power and executed a forced landing to an open field. The airplane sustained substantial damage to the left horizontal stabilizer. The instructor reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation and that he should have assumed control of the airplane sooner.