ARTCC RADAR CTLR RECEIVING OJT CLEARED SMA TO OCCUPIED ALT INADVERTENTLY; INSTRUCTOR CTLR DID NOT SEE ERROR UNTIL CONFLICT ALERT ACTIVATED.

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I WAS GIVING RADAR OJT. THE SECTOR WAS QUITE BUSY WITH ACFT TRANSITIONING TO RDU. AN ACFT (SMA X) WAS CLRED TO DEPART LYH AND PROCEED SWBND AT 4000'. AFTER RADAR IDENT SMA X WAS CLRED TO 7000' INADVERTENTLY. THE DATA BLOCK WAS CHANGED TO REFLECT 6000'. I DIDN'T NOTICE THIS BECAUSE I WAS PREOCCUPIED WITH OVERALL SECTOR. SMA Y EBND OVER LYH WAS TFC AT 7000'. RADAR OJT THOUGHT HE HAD ISSUED 6000'. I FIRST NOTICED SMA X AT 6400'. I ASKED ACFT WHAT HIS ALT WAS AND HE REPLIED 6800' I TOLD HIM TO MAINTAIN 6000' AND THAT WAS HIS ASSIGNED ALT. HE ADVISED HE HAD BEEN ASSIGNED 7000' AND READ BACK 7000'. HE ALSO ADVISED HE SAW TFC THAT APPEARED TO BE AT 7000'. I TOLD HIM IT WAS AT 7000'. TARGETS WERE NOW ON OPP COURSES. SMA X ASKED IF I STILL WANTED HIM TO DSND TO 6000'. I TOLD HIM NOT NECESSARY AND CONTINUED CLB. MANUAL CTLR ADVISED HE HEARD CLRNC TO 7000' AND ADVISED RADAR OJT. RADAR OJT THOUGHT MANUAL CTLR SAID; 'CLB HIM TO 7000'.' I DIDN'T HEAR THIS EITHER. I INFORMED MANUAL CTLR THAT HE SHOULD HAVE INFORMED ME THAT I WAS THE PERSON RESPONSIBLE FOR THE OPERATION. HE WAS NOT THE CAUSE OF THE SITUATION BY ANY MEANS; BUT COULD HAVE PREVENTED IT IF HE HAD TOLD ME. MANUAL CTLR WAS RECENTLY CERTIFIED AND IS STILL A DEVELOPMENTAL CTLR. THE SECTOR (R30 VALLEY LOW) WAS VERY BUSY AT THE TIME AS IT IS EVERY DAY AT THIS TIME XA10-XA45Z WITH TFC BEING VECTORED TO LAND AT RDU. FOLLOWING ARE THINGS THAT I FEEL COULD HAVE HELPED TO AVOID SITUATION. 1) NO DEVELOPMENTAL (MANUAL) CTLRS WHILE RADAR OJT IN PROGRESS (ESPECIALLY AT PEAK PERIODS). 2) SECTOR FULLY STAFFED AT PEAK PERIODS. 3) METERING OF ACFT FROM THEIR DEP POINTS TO AVOID IMPACT. 4) MORE STAFFING OVERALL. THIS INCIDENT IS MY FAULT AND I ACCEPT THE BLAME FULLY. MY INATTN TO THESE PERTINENT DETAILS IS OBVIOUSLY THE OVERALL CAUSE OF THE INCIDENT. MY OJT WAS SEPARATING TFC FROM AN ACFT AT 7000' AND INADVERTENTLY SAID THE ALT OF THE CONFLICTING ACFT. I DIDN'T CATCH ON READBACK EITHER.