DURING TOUCH AND GO; TRAINEE PLT INADVERTENTLY HIT GEAR SWITCH WHEN PUSHING IN CARB HEAT. WHEN WT OF THE ACFT BEGAN COMING OFF THE GEAR; IT CAME UP WITH PROP STRIKING RWY. INSTRUCTOR PLT TOOK OVER AND FLEW THE ACFT BACK TO ITS BASE.

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I WAS CONDUCTING A FLT LESSON WITH A CFI APPLICANT WITH THE APPLICANT SITTING IN THE R SEAT OF A C172RG. WE DEPARTED TTD AT XA45 AND FLEW 14 NM S-SE TO S48 TO CONDUCT A LESSON ON SHORT FIELD LNDG PROCS. I WAS FLYING THE ACFT AND DEMONSTRATED A SHORT FIELD APCH WITH A NORMAL TOUCHDOWN AND CONTINUED THE 'TOUCH AND GO' TO HAVE THE APPLICANT DEMONSTRATE THE SAME PROC. THE APPLICANT THEN DEMONSTRATED THE SAME PROC WITH A SHORT FIELD APCH AND LNDG. IT WAS DURING THE TKOF PORTION OF THE PROC WHEN THE PROB AROSE. DURING THE TKOF RUN; I WAS FOCUSING ON CTRLINE CTL AND AIRSPD. AS THE ACFT APCHED ROTATION SPD; I NOTICED THE NOSE DROPPING BELOW ITS NORMAL POS AND IMMEDIATELY GRABBED THE CTLS TO BRING THE NOSE BACK UP. I HEARD A SCRAPING SOUND AND KNEW THAT IT MUST HAVE BEEN THE PROP STRIKING THE ASPHALT; BUT WE WERE ABOUT OUT OF RWY AT THIS POINT; SO I CONTINUED THE TKOF. DURING THE CLB OUT; THERE WAS NO VIBRATION AND THE ENG WAS PRODUCING SUFFICIENT PWR TO CLB; SO I CONTINUED TO CLB. ONCE THE ACFT REACHED A SAFE ALT; I HAD MORE TIME TO THINK ABOUT OUR OPTIONS. AT THIS POINT; THE ENG CONTINUED TO PRODUCE SUFFICIENT PWR WITHOUT VIBRATION SO I CHOSE TO FOLLOW LOWER FLATTER TERRAIN BACK TO TROUTDALE AND LANDED NORMALLY. EVIDENTLY; AS THE APPLICANT BROUGHT THE FLAPS UP AND PUSHED THE CARB HEAT IN; HE ALSO SELECTED GEAR UP. DURING THE TKOF RUN; THE SQUAT SWITCH KEPT THE GEAR FROM RETRACTING UNTIL THERE WAS ENOUGH WT OFF OF THE NOSE. I FEEL THAT MY ACTIONS WERE VERY APPROPRIATE FOR THE SIT BECAUSE IF I WOULD HAVE ALLOWED THE ACFT BACK DOWN ONTO THE GND; WE WOULD HAVE ENDED UP IN THE TREES AT THE END OF THE RWY. TO PREVENT THIS TYPE OF OCCURRENCE FROM HAPPENING IN THE FUTURE I WOULD: 1) PROTECT THE GEAR SELECTOR ANY TIME THE GEAR IS EXTENDED; AND 2) CHOOSE TO ONLY DO STOP AND GO'S IN ACFT WITH RETRACTABLE GEAR. I FEEL THAT FACTORS THAT MAY HAVE CONTRIBUTED TO THIS SIT MAY HAVE BEEN: 1) THE APPLICANT DID NOT IDENT AND VERIFY THE CTLS BEFORE HE ENGAGED THEM.