CONTRARY TO ATC CLRNC; INSTRUCTOR PLT WITH AN INST STUDENT ENTERS THE WRONG TFC PATTERN ON DOWNWIND LEG.

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MANCHESTER APCH CLRED MY STUDENT FOR A VISUAL APCH APPROX 12 MI NW OF NASHUA'S RWY 14/32. UPON CONTACTING NASHUA TWR THE STUDENT RPTED TO THE SW. WE WERE ASKED TO RPT A MIDFIELD L DOWNWIND FOR RWY 32. I CORRECTED THE STUDENT AND HE UPDATED HIS POS AS NW. WE WERE ALSO ADVISED OF OTHER TFC APCHING THE L DOWNWIND. I BEGAN FLYING THE ACFT AT THIS TIME TO ALLOW THE STUDENT TIME TO ORIENT HIMSELF. I FLEW THE EXTENDED CTRLINE UNTIL ABOUT 4 MI FROM THE DEP END OF RWY 32. I THEN TURNED TOWARD THE L DOWNWIND. LESS THAN A MILE LATER THE STUDENT INSISTS WE ARE ENTERING THE WRONG DOWNWIND. ASSUMING THE STUDENT MIGHT BE LISTENING MORE CLOSELY; I MOVED TO THE R DOWNWIND WHICH IS THE TWR PREFERRED PATTERN. I RETURNED CTL TO THE STUDENT PRIOR TO A MIDFIELD R DOWNWIND BUT (UNKNOWN TO ME AT THE TIME) HE RPTED A MIDFIELD L DOWNWIND. THE OTHER TFC ALSO RPTED A MIDFIELD L DOWNWIND. THE TWR COULDN'T SEE US NOR THE OTHER ACFT. A TENSE EXCHANGE WITH BOTH ACFT LASTED ABOUT 2 MINS BY WHICH TIME OUR TFC WAS ON SHORT FINAL AND WE WERE TURNING A WIDE (TO AVOID UNSEEN TFC) BASE. I TEACH PLTS TO BE WILLING TO ACCEPT CHALLENGES. I MUST ALSO REMEMBER SOMETIMES THOSE CHALLENGES ARE WRONG.