INSTRUCTOR WITH SPI USES WRONG XPONDER CODE ENTERING CLASS B AIRSPACE.

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ON AN INSTRUCTIONAL FLT A PROB OCCURRED WITH A WRONG XPONDER CODE BEING PLACED IN THE XPONDER. THE FLT ORIGINATED AT HEBER MUNICIPAL WITH TOUCH-AND-GOES AT PROVO MUNICIPAL AND THEN TO SALT LAKE; #2 (U42). THE PROB WAS REALIZED WHEN TRYING TO TRANSITION THROUGH CLASS B AIRSPACE TO A SATELLITE ARPT CLOSE BY. AT THE START OF THE FLT WE FOLLOWED THE CHKLIST AS USUAL FOR RUN-UP AND FOR PREPARATION TO TKOF. USUALLY I HAVE ALWAYS HAD CHKLISTS THAT SAY XPONDER TO 1200 AND STANDBY. I DID NOT THINK ABOUT CHKING THE XPONDER BECAUSE I ALREADY HAD ENOUGH THINGS GOING ON IN THE AIRPLANE. THE STUDENT PLT AND I THEN TRAVELED 30 MI THROUGH A CANYON TO PROVO MUNICIPAL AND DID TOUCH-AND-GOES. I HAVE FLOWN WITH THE STUDENT PLT FOR 10-15 HRS AND KNOW THAT I HAVE TO HAVE TO STAY ON TOP OF THE AIRPLANE. (SHE HAD 70 HRS OF INSTRUCTION BEFORE SHE ASKED ME TO INSTRUCT HER FOR HER PVT LICENSE). THIS WAS THE STUDENT PLT'S SECOND TIME IN THE CLASS B AIRSPACE. I HAD BRIEFED HER BEFORE WE TOOK OFF; AND ASSUMED I WOULD JUST HELP IF ANY PROBS AROSE. WHEN THE CTLR STARTED CALLING OUR IDENT; ALL I CAUGHT WAS THE 'YZ' AND NOTHING ELSE. I THEN SWITCHED RADIOS AND ASKED THE STUDENT TO ASK ATC TO SAY AGAIN. THEY THEN ASKED IF WE WERE IN THE PLANE AT THE POINT OF THE MOUNTAIN AND WHY WERE WE SQUAWKING 7700. I WAS CAUGHT QUITE OFF GUARD AND SWITCHED THE XPONDER TO 1200. I DID NOT MAKE A COMMOTION OUT OF THE PROB BECAUSE I KNEW THE STUDENT WAS NERVOUS AS IT WAS. (WE TALKED ABOUT THE PROB WHEN WE GOT ON THE GND THOUGH.) THE STUDENT PLT AND CTLR THEN CONTINUED TO GO ON TALKING AND GOT US INTO THE CLASS B AIRSPACE AFTER TELLING THE CTLR THAT SHE WAS A STUDENT PLT AND WANTED TO TRANSITION. DURING ALL OF THIS I NEVER SAID A WORD ON THE RADIO BECAUSE IT WAS BEING HANDLED. I KNOW THIS IS A SERIOUS OFFENSE AND FEEL VERY BADLY ABOUT THIS. I KNOW BETTER THAN TO HAVE THIS PROB AGAIN; AND IT WILL NEVER HAPPEN AGAIN. THIS EVENING I WAS INFORMED THE STUDENT'S HUSBAND WAS SHOWING A FRIEND DIFFERENT THINGS THE NIGHT BEFORE; AND FORGOT TO TURN THE XPONDER BACK TO 1200. THINGS LEADING UP TO THIS ARE: I REALIZE IT WAS GETTING TIME TO GO HOME I MAY HAVE BEEN A LITTLE TIRED AND THE LITTLE TIME SPENT WITH THE CHKLIST FOR THIS AIRPLANE ITSELF. I HAVE NEVER DONE ANYTHING LIKE THIS BEFORE AND DO NOT PLAN TO DO ANYTHING IN THE FUTURE LIKE THIS. I AM TRULY SORRY IF I HAVE CAUSED PROBS AND INTEND TO NEVER MAKE THE SAME MISTAKE. AS FAR AS FOLLOWING CHKLISTS GO; I PLAN TO WRITE ON THE CHKLIST IN THAT AIRPLANE AND ANY OTHER CHKLIST THAT DOES NOT HAVE THE 1200 CODE ON THEM. THIS HAPPENED TO BE A CESSNA 182 CHKLIST THAT CAME WITH THE AIRPLANE.