INSTRUCTOR WITH STUDENT PLT IN A C172 HAS NMAC WITH ANOTHER C172 WHO DID NOT ANNOUNCE POS ON FINAL AT ARPT WITH TWR NOT YET OPEN IN EARLY AM. A VOICE ON TWR FREQ GIVES ERRONEOUS INFO REGARDING THE GAR TFC. TWR OPERATES BY NOTAM.

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AT APPROX AM30 HRS; 2 OR 3 ACFT WERE ALREADY AIRBORNE WITH AT LEAST 2 ACFT REMAINING IN THE PATTERN AT BLACKSTONE/FT PICKETT (BKT). MY STUDENT HAD COMPLETED THE PREFLT RUN-UP AND HAD TAXIED TO THE HOLD AREA AT THE END OF RWY 22. I (AND MY STUDENT) CHKED THE FINAL APCH TO RWY 22 AND OBSERVED NO ACFT ON FINAL APCH. I THEN HAD MY STUDENT TAXI INTO POS ON RWY 22 AND ANNOUNCE 'CESSNA TAKING OFF ON RWY 22; CLOSED PATTERN.' AS WE LIFTED OFF ANOTHER ACFT ANNOUNCED 'GOING AROUND -- RWY 22' AND I THINK HE MAY HAVE ALSO SAID 'EXECUTING MISSED APCH.' I'M SURE THERE WAS MORE TO THE MESSAGE THAN THAT BUT ALL I REMEMBER FOR SURE WAS 'GOING AROUND.' I THEN OPENED THE MIKE AND SAID 'WHERE IS THE ACFT GOING AROUND?' OR I MAY HAVE SAID 'IS THERE AN ACFT GOING AROUND?' AN INDIVIDUAL USING THE TWR FREQ ANNOUNCED (THE TWR WAS NOT OPERATING) ON THE FREQ 'THERE IS NO ACFT GOING AROUND' AND I THINK HE ALSO SAID; 'YOU ARE CLR TO TURN XWIND.' THIS WOULD BE A R TURN AS THE L SIDE OF THE PATTERN FOR RWY 22 ABUTS A RESTR AREA (WHICH CONTAINS ACTIVE MIL OPS); THUS ANY ACFT 'GOING AROUND' WOULD HAVE MADE A SIDESTEP TO THE R (BECAUSE OF THE RESTR AREA AND BECAUSE THIS IS HOW THE PROC IS OUTLINED IN THE AERONAUTICAL INFO MANUAL).' I TOOK NO IMMEDIATE ACTION PRIMARILY BECAUSE I DID NOT TRUST THE INFO I HAD JUST BEEN GIVEN. PERHAPS 30 SECONDS LATER THE PLT OF THE ACFT EXECUTING THE MISS (GOING AROUND) ANNOUNCED THAT HE WAS NOW IN TRAIL WITH US AND WE COULD TURN R. THE NEXT ANNOUNCEMENT FROM THE TWR (A MIN OR TWO LATER) WAS A DIFFERENT PERSON. I DID NOT HEAR THE ORIGINAL (FIRST) VOICE ON THE TWR FREQ AGAIN. CONCLUSION: UNAUTH USE OF FREQS CAN POTENTIALLY BE CATASTROPHIC. IF YOU ARE UNSURE OF A RESPONSE; ASK AGAIN. RPT YOUR POS IN THE PATTERN. HAD THE ACFT THAT MADE THE 'GAR' RPTED DOWNWIND; TURNING BASE; OR BASE; I AND MY STUDENT WOULD NOT HAVE TAXIED ONTO THE RWY.