ACR B757 DEPARTED WITH 2 FLT ATTENDANTS STILL STANDING. CREW MEMBERS WERE NOT AT THEIR DUTY STATIONS DURING TKOF.

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WE WERE #1 FOR TKOF AT MEX; BUT THE FLT ATTENDANT SAFETY TAPE WAS STILL RUNNING. AFTER TAPE WAS COMPLETE GAVE PREPARE FOR TKOF PA AND RANG CHIME. ABOUT 30 SECONDS LATER; WE BEGAN TKOF ROLL. AFTER LEVEL OFF; A FLT ATTENDANT CAME TO THE COCKPIT AND SAID 2 FLT ATTENDANTS WERE NOT SEATED WHEN ROLL BEGAN AND IT WAS UNSAFE. I ASKED IF THEY HEARD THE PA AND CHIME. SHE SAID THEIR PROCS WERE TO REMAIN SEATED UNTIL AFTER TAPE WAS FINISHED (AS NOT TO BLOCK THE PAX VIEW OF THE TAPE) AND THEN DO A SAFETY CHK WHICH CONSISTED OF SEAT BELTS; TRAYS; ETC. I SHOWED HER OUR REQUIREMENTS AND ASKED TO SEE HER MANUAL. SHE SHOWED IT AND IT DID SAY NOT TO BEGIN THE TAPE UNTIL FORWARD MOTION OF THE ACFT; BUT DID NOT SAY OR ADDRESS THE STAY SEATED UNTIL TAPE FINISHED. SHE SAID THAT WAS WHAT WAS TAUGHT; AND ALL FLT ATTENDANTS DID THAT. SINCE WE HAVE NO MECHANISM TO KNOW WHEN THEY ARE FINISHED; PLUS OUR COMPANY SPENDS MORE TIME ON THE TXWY WAITING; RECOMMEND THAT THE FLT ATTENDANTS CHANGE THEIR PROCS TO DO THE SAFETY CHK WHILE THE TAPE IS RUNNING. AS IT STANDS NOW THERE IS A CONSIDERABLE CHANCE FOR A FLT ATTENDANT INJURY OR THE REQUIRED SAFETY CHK IS BEING DISCONTINUED WHEN THE CHIME IS HEARD.