AFTER DEPARTING W48 WITH A STUDENT UTILIZING A LCL ARPT TFC PATTERN XPONDER SQUAWK; INSTRUCTOR IS ALERTED BY PCT OF IMPROPER DEP PROCS INTO THE DC ADIZ.

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I PLANNED AN INST TRAINING FLT WITH A STUDENT; FROM W48 (ESSEY; MD); TO MRB (MARTINSBURG; WV). THE STUDENT FILED AN INST FLT PLAN AND ASKED THE BRIEFER WHAT FREQ TO PICK UP THE IFR CLRNC ON. THE BRIEFER TOLD MY STUDENT 119.0. WE DEPARTED W48 WITH A XXXX SQUAWK AND CONTACTED POTOMAC APCH AROUND 800 FT AGL. AFTER A MIN OR TWO; POTOMAC GAVE US OUR IFR CLRNC. WE WERE THEN GIVEN AN 800 NUMBER TO CALL. I HAD WRONGLY ASSUMED THAT; BECAUSE W48 LIES WITHIN MTN (MARTIN STATE; BALTIMORE; MD) CLASS D AIRSPACE; THAT WE COULD DEPART W48 ON A XXXX SQUAWK AND CONTACT APCH FOR OUR IFR CLRNC ONCE AIRBORNE. AT THE TIME; IT WAS NOT CLR TO ME HOW TO PICK UP AN IFR CLRNC FROM W48 (OTHER THAN ON THE GND); SINCE THE ADIZ WAS SURROUNDING THE BALTIMORE AND WASHINGTON AREA. WHEN THE STUDENT ASKED THE BRIEFER FOR THE FREQ TO PICK UP AN IFR FLT PLAN; THE BRIEFER SAID 119.0; BUT DID NOT MENTION THAT YOU FIRST HAVE TO AT LEAST GET A SQUAWK ON THE GND TO ENTER THE ADIZ; (WHICH BOTH THE STUDENT AND I SHOULD HAVE REALIZED). THIS FURTHER LED TO OUR CONFUSION. ACCORDING TO MY STUDENT (WHO GOT THE BRIEFING); THE BRIEFER DID ASK IF HE WAS FAMILIAR WITH THE ADIZ; WHICH HE SAID HE WAS. I WAS ALSO LED INTO BELIEVING THAT DEPARTING W48 ON A XXXX SQUAWK WAS OK; BECAUSE MANY OTHER PLTS WERE DOING IT FROM W48; AND MY STUDENT HAD SAID HE HAD DONE IT BEFORE. I SHOULD HAVE CONFIRMED WITH FSS OR APCH HOW TO PROPERLY DEPART W48 AND NOT RELIED UPON LCL PLTS INTERP OF THE NOTAMS. I ALSO SHOULD HAVE GOTTEN A BRIEFING MYSELF; INSTEAD OF HEARING IT SECOND HAND FROM THE STUDENT. IN THE FUTURE; I WILL MAKE SURE TO READ THE NOTAM MYSELF; AND IF IT IS NOT CLR; I WILL GET CLARIFICATION FROM EITHER FSS OR ATC; NOT LCL PLTS.