A C172 PLT AND SAFETY PLT NEGLECTED TO SQUAWK THE PROPER TRANSPONDER CODE; SPAWNING A FIGHTER INTERCEPT.

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ON 11/SAT/01; I WAS ACTING AS A SAFETY PLT ON A CESSNA 172 (XXXX). WE FILED AN IFR FLT FROM ZZZ TO ZZZ1 USING MY NAME. THE FLT WAS INTENDED TO ALLOW THE OWNER TO PREPARE FOR HIS INSTRUMENT CHECK RIDE WHICH WAS PLANNED FOR A WEEK LATER. WE WERE PERFORMING MULTIPLE PRACTICE INSTRUMENT APCHS AT ZZZ1 ARPT. THE WX WAS CLR AND THE PATTERN QUICKLY BECAME FULL OF INCOMING AND DEPARTING TFC. WE WERE TOLD BY ZZZ2 APCH CTL THEY COULD NO LONGER PROVIDE RADAR SVC DUE TO THE VOLUME OF TFC. WE ASKED TO TERMINATE OUR IFR FLT PLAN AND ZZZ2 TOLD US TO SQUAWK VFR AND RADAR SVC TERMINATED. WE EXITED ZZZ1 AFTER OUR LAST APCH AND PUSHED THE BENDIX KING KT-76C TRANSPONDER'S VFR BUTTON TO SQUAWK 1200; APPARENTLY THE BUTTON DID NOT TRANSFER THE SQUAWK CODE FROM XXXX TO THE 1200 VFR CODE. WE PROCEEDED APPROX 150 MILES SE OF ZZZ2; 10 MILES S OF ZZZ AND CLIMBED TO 4500 FT MSL TO DO SOME AIRWORK. WE PERFORMED A SERIES OF PRACTICE MANEUVERS. IT WAS ON THE DME ARC WE NOTICED A MIL F18 CLIMBING OFF OUR R WING. HE WAS APPROX 1 TO 2 MILES OFF OUR WING. WE CHECKED OUR POSITION IN RELATIONSHIP TO THE MOA TO ENSURE WE WERE CLR OF THEIR AIRSPACE. WE WERE MANY MILES TO THE W OF THEIR AIRSPACE. WE TURNED TOWARD ZZZ AND WERE IN THE PROCESS OF DESCENDING TO THEIR PATTERN ALTITUDE WHEN THE F18 INTERCEPTED US. HE WAS LEVEL AT OUR ALTITUDE LESS THAN A HALF MILE OFF OUR L WING. WE MOTIONED TO HIM WITH A THUMB DOWN WE WERE LNDG AT ZZZ ARPT. THE F18 ACCELERATED AND PERFORMED A CLIMBING L HAND TURN. WE WERE MONITORING 121.5 AFTER AND HEARD NOTHING OVER THE RADIO. ZZZ TWR INFORMED US TO CALL ZZZ3 CTR WHEN WE GOT ON THE GND. WE WILL BE MORE VIGILANT ABOUT OUR TRANSPONDER CODES IN THE FUTURE. SUPPLEMENTAL INFO FROM ACN 529942: THE BENDIX KING KT-76C TRANSPONDER HAS A BUTTON WHICH ALLOWS ME TO CANCEL THE ASSIGNED CODE AND TO TRANSMIT 1200. IN THE FUTURE I WILL BE MORE CAREFUL TO CONFIRM THAT I AM IN FACT USING THE PROPER CODE.