INSTRUCTOR WITH SPI TAXIES ACROSS ACTIVE RWY WITH ACFT TAKING OFF BELIEVING HE HAD CLRNC. HE DID NOT.

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WHILE HOLDING SHORT OF RWY 7L ON TXWY E; HEADED BACK TO THE RAMP; I BELIEVED TO BE CLRED TO CROSS THE ACTIVE RWY. I ACKNOWLEDGED GND CTL AND BEGAN TO TAXI ACROSS; GIVING WAY TO THE OPPOSITE DIRECTION TFC. I SOON NOTICED THAT THE OTHER ACFT WERE STILL HOLDING SHORT AND THAT AN ACFT HAD JUST TAKEN OFF. I IMMEDIATELY TAXIED CLR OF THE RWY AND AWAITED FURTHER TAXI INSTRUCTIONS. THE OPPOSITE DIRECTION TFC ALSO APPEARED TO BE CONFUSED AND HAD ASKED GND CTL IF THEY WERE CLRED TO CROSS. ALL TFC WAS INSTRUCTED TO HOLD SHORT MOMENTARILY FOLLOWED BY CLRNCS TO CONTINUE TAXI. IN RETROSPECT; I BELIEVE THAT IT IS POSSIBLE THAT; IN FACT; I MAY HAVE NOT ACTUALLY BEEN CLRED TO CROSS RWY 7L. ALTHOUGH; AT THE TIME OF THE OCCURRENCE; I WAS CERTAIN TO BE CLRED! CONTRIBUTING FACTORS: WHILE HOLDING; WAS DEBRIEFING STUDENT ON PERFORMANCE. THIS OBVIOUSLY INTERFERED WITH MY ABILITY TO HEAR ATC. DID NOT SEE ACFT TAKING OFF BECAUSE I WAS WATCHING THE OPPOSITE DIRECTION TFC ON THE SAME TXWY. DUE TO THE MANY ACFT XMITTING ON GND CTL; I BELIEVE MY ACKNOWLEDGEMENT TO ATC WAS MERELY 'ROGER' INSTEAD OF 'XING RWY 7L.'