ACFT CROSSES RWY THAT IS ACTIVE AND COMES WITHIN 1000 FT OF DEPARTING ACFT ON ITS TKOF ROLL.

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UPON FINISHING A TRAINING FLT; AND TAXIING TO THE FBO (BASE OF OPS); WE WERE TOLD TO HOLD SHORT OF RWY 25L. AFTER A FEW MOMENTS WE WERE TOLD TO CROSS RWY 25L AND HOLD SHORT OF RWY 25R. HOWEVER; I DIDN'T HEAR IT AND MY STUDENT JUST READ BACK THE LAST 3 LETTERS. AS WE CROSSED RWY 25L I KNEW I DIDN'T HEAR CLRLY WHAT GND SAID AND I QUESTIONED MY STUDENT; 'DID WE GET CLRED TO CROSS RWY 25R?' NO RESPONSE. AGAIN I ASKED; NO RESPONSE. WE WERE APCHING THE HOLDLINE TO RWY 25L AND I LOOKED AND SAW A TWIN IN POS AND HOLD ON RWY 25R BUT I THOUGHT THE SIT WASN'T GOOD. I; FOR ONLY A MOMENT; CONSIDERED USING THE BRAKES; BUT I DIDN'T BECAUSE MY STUDENT STARTED TO USE THEM. BUT HE DIDN'T STOP. HE ROLLED RIGHT PAST THE HOLDLINE. BY THAT TIME I SAW THAT THE ACFT IN POS WAS NOW MOVING DOWN RWY 25R. I FIGURED BY THEN IT WAS TOO LATE FOR ME TO BRAKE SO I TOOK CTL AND MOVED ACROSS QUICKLY WHILE GND TOLD US THAT WE WERE SUPPOSED TO HOLD SHORT. THE PROB AS I SEE IT WAS ME NOT HEARING THE CROSS AND HOLD SHORT CALL THEN MY STUDENT JUST READING BACK THE CALL SIGN THEN GND CTL NOT ASKING US TO CONFIRM HOLD SHORT (READBACK; THAT IS). THEN ME THINKING FOR A FEW TOO MANY MOMENTS THAT WE MAY HAVE BEEN CLRED AND TOO LATE DECIDING THAT THAT COULDN'T BE. THEN CONCLUDING ONLY AFTER XING THE HOLDLINE THAT THINGS DIDN'T LOOK RIGHT; BUT THEN HAVING TO DECIDE TO CARRY THROUGH THE TRANSITION ACROSS RWY 25R WITH ALL SPD AND REGRET. I'M QUITE AWARE OF COMPLACENCY; IT IS INSIDIOUS. ALTHOUGH MY STUDENT HAS HIS PVT -- IT WAS MY RESPONSIBILITY.