FLT INSTRUCTOR AND STUDENT PLT DO EVERYTHING RIGHT IN THE DC ADIZ BUT FORGET TO SQUAWK ALT MODE ON XPONDER. GOT CAUGHT.

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I HAD FOLLOWED ALL ADIZ PROCS BY FILING AN ADIZ FLT PLAN; RECEIVING XPONDER CODE FROM POTOMAC CLRNC; CONTACTING POTOMAC DEP AFTER DEP FROM JYO AT ABOUT 1200 FT MSL. POTOMAC DEP ADVISED ME TO SET XPONDER TO MY ASSIGNED CODE. AT THAT TIME; I NOTICED THE XPONDER WAS STILL ON STANDBY; INSTEAD OF ALT MODE. I IMMEDIATELY TURNED IT TO ALT MODE AND POTOMAC ADVISED OF RADAR CONTACT. ABOUT 15 MINS LATER ON THE RETURN TO JYO; THE CTLR SAID THE SUPVR WAS ASKING WHY THEY ONLY HAD A 'PRIMARY' TARGET FOR ME ON THE DEP. I PROVIDED THE REASON; AND A FEW MINS LATER HE PROVIDED A PHONE NUMBER TO CALL UPON LNDG. I CALLED THE PHONE NUMBER UPON LNDG. I EXPLAINED WHAT HAPPENED. HE SAID HE WOULD INVESTIGATE THE EVENT AND DISCUSS WITH THE CTLR TO DETERMINE IF THIS WAS A PLTDEV. IF IT IS DETERMINED TO BE A DEV; I WOULD HEAR FROM THE FSDO. I WAS INSTRUCTING A STUDENT WHO HAD NEVER FLOWN AT NIGHT AND WAS NOT FAMILIAR WITH THE ACFT; NOR THE JYO AIRSPACE. DUE TO THE INSTRUCTING; I WASN'T ABLE TO CALL FOR A XPONDER CODE UNTIL AFTER THE STUDENT COMPLETED THE BEFORE TKOF CHKLIST; WHICH INCLUDES THE XPONDER TO ALT MODE. WHEN I RECEIVED THE CODE; I WAS DISTR BY THE STUDENT AND NEVER TURNED THE XPONDER FROM STANDBY TO ALT MODE.