INSTRUCTOR AND STUDENT RECEIVE A CLRNC TO TAXI INTO POS AND HOLD THEN NOTICE TFC ALREADY IN POS FURTHER DOWN THE RWY.

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I WAS WITH MY STUDENT FOR HIS FIRST FLT IN INST. WE TAXIED TO THE HOLD SHORT LINE AND CALLED TWR. THEY TOLD US TO HOLD SHORT. I RESPONDED 'SKYHAWK X HOLDING SHORT.' I NOTICED A PLANE ON SHORT FINAL AND HEARD A PLANE IN THE DOWNWIND. THE PLANE ON SHORT FINAL LANDED AND A MIN OR TWO WENT BY AND I HEARD THE TWR SAY 'SKYHAWK XX; POS AND HOLD.' I CHKED THE FINAL AND I SAW A PLANE ON A L BASE FOR THE RWY; SO I FIGURED THE TWR WAS PUTTING US INTO POS TO TKOF BEFORE THE PLANE ON BASE WOULD LAND. MY STUDENT RESPONDED 'XX; POS AND HOLD.' A FEW SECONDS WENT BY WHILE MY STUDENT DID HIS LINE-UP CHKLIST AND WE PROCEEDED TO TAXI ONTO THE RWY. WHEN MY STUDENT TURNED TOWARDS THE RWY BECAUSE OUR ACFT WAS FACING FINAL; SO I COULD SEE IF THERE WERE ANY PLANES COMING. HE TURNED TO THE L AND I SAW A PLANE ABOUT 2000 FT DOWN ON THE RWY CTRLINE FACING AWAY FROM US HOLDING POS. I STEPPED ON THE BRAKES AND CALLED TWR TO VERIFY THAT WE WERE CLRED TO POS AND HOLD. THE TWR SAID 'NEGATIVE.' I INFORMED THEM OF MY POS. THEY TOLD ME TO CONTINUE TAXIING TO THE RWY TO POS AND HOLD THERE. THE PLANE THAT WAS ON THE BASE LEG HAD NOW TURNED FINAL AND HE CLRED THE AIRPLANE THAT WAS 2000 FT DOWN THE RWY TO TKOF AND TOLD THE PLANE ON FINAL TO GO AROUND. WE SAT ON THE RWY FOR A MIN OR TWO AND THEN WE WERE CLRED TO TKOF. I ASKED MY STUDENT WHAT HE HEARD THE TWR SAY; AND HE CONFIRMED THAT THEY SAID TO POS AND HOLD. I CONTACTED TWR AFTERWARD AND THEY SAID THERE WASN'T ANY VIOLATION. I THINK THIS COULD HAVE BEEN PREVENTED BY ATC SPEAKING MORE CLRLY AND ANSWERING BACK WHEN I RESPONDED WITH 'XX POS AND HOLD.' I WILL FROM NOW ON AND I WILL NOT ABBREVIATE MY TAIL NUMBER EVEN IF ATC DOES AND I WILL QUERY ATC IF AT ANY TIME THERE IS A PLANE ON BASE OR HOLDING SHORT OF THE RWY. THE FLT SCHOOL I'M ASSOCIATED WITH IS INVOLVED IN THE FAA ASAP PROGRAM. I FILLED OUT THAT RPT JUST LIKE I AM SUPPOSED TO.