C172 PLT HAS WRONG XPONDER CODE SELECTED.

Narrative

ON AN INTRODUCTORY FLT FOR A NEW STUDENT; A PORTION OF THE FLT WAS INADVERTENTLY MADE WITH THE XPONDER SET TO RESPOND TO A CODE OTHER THAN 1200. THE ACFT WAS EQUIPPED WITH THE GARMIN G1000 EFIS. WHILE ATTEMPTING TO TEACH CHKLIST USAGE TO A NEW STUDENT; THE ONLY COPY OF THE CHKLIST WAS IN THE STUDENT'S POSSESSION AND I AS THE INSTRUCTOR FAILED TO REALIZE THAT THE XPONDER SETTING ITEM HAD BEEN SKIPPED. SINCE THE XPONDER AUTOMATICALLY ACTIVATES IN ALT MODE ONCE THE ACFT BEGINS THE TKOF ROLL; AND THIS FEATURE HAD BECOME RELIED UPON FOR XPONDER ACTIVATION; NO INTERACTION WITH THE XPONDER WAS INITIATED BY ME RESULTING IN AN INADVERTENT TKOF AND PERIOD OF FLT WHILE SQUAWKING AN INAPPROPRIATE XPONDER CODE. THIS OCCURRENCE EMPHASIZES THE NEED FOR PROPER CHKLIST USAGE. WHILE ATTEMPTING TO TEACH THIS TO A NEW STUDENT; I FAILED TO UTILIZE THE CHKLIST MYSELF. IF I HAD A SECOND COPY OF THE CHKLIST FOR MY OWN USE; AN ADEQUATE REVIEW OF THE CHKLIST PROCS COULD HAVE BEEN PERFORMED AND THE INAPPROPRIATE SQUAWK CODE CORRECTED BEFORE THE TKOF ROLL. IT ALSO EMPHASIZES THE NEED TO MONITOR AUTOMATED SYS CLOSELY. IN THIS CASE; RELYING UPON THE AUTOMATIC ACTIVATION OF THE XPONDER REMOVED PLT INTERACTION WITH THAT SYS AND ALLOWED THE ERROR TO GO UNNOTICED.