The instructional flight was practicing stop-and-go takeoffs and landings. On the last landing, the pilot under instruction failed to put the landing gear handle down. Power was maintained until the runway due to the wind condition so the landing gear warning horn did not sound. The instructor pilot did not see that the landing gear lights were not illuminated, and did not realize that the landing gear handle was not down until runway contact. The firewall sustained substantial damage during the gear-up landing.