A PA32 INSTRUCTOR PLT ALLOWS HIS STUDENT TO STRAY OFF AN ASSIGNED HEADING; CREATING A TFC CONFLICT DURING A VECTORED APCH TO SNA; CA

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ON THE MORNING OF JUL/FRI/99; I DEPARTED BURBANK WITH MY 2 STUDENTS ON A TWR ENRTE CLRNC TO JOHN WAYNE/SANTA ANA ARPT. THE WX WAS VMC WITH UNLIMITED CEILING; AND GOOD VISIBILITY. APPROX 15 MI OUT; SOCAL APCH BEGAN VECTORING US FOR THE ILS RWY 19 APCH. THE STUDENT WAS FLYING THE APCH AS I MONITORED THE PROGRESS. THE LAST ATC COMMAND WAS TO FLY PRESENT HEADING; VECTORS TO LOC. AT THIS POINT I TURNED MY ATTN TO MY APCH PLATES FOR THE NEXT DEP RTE WE WOULD BE REQUESTING WHEN ON THE GND. I ESTIMATE I HAD MY HEAD DOWN FOR 15 SECONDS. DURING THIS TIME MY STUDENT INITIATED A L TURN OF APPROX 30 DEGS. WHEN I LOOKED BACK UP THE AIRPLANE WAS LEVEL. I DID NOT DETECT THE HEADING CHANGE. SEVERAL MOMENTS LATER SOCAL APCH ISSUED A COMMAND TO TURN IMMEDIATELY R; 260 DEG HDG. THE L TURN THAT MY STUDENT INITIATED PUT US ON AN INTERCEPT HEADING FOR THE LOC IN DIRECT CONFLICT WITH OTHER TFC. THERE WERE; IN MY OPINION; SEVERAL FACTORS LEADING UP TO THIS INCIDENT. ALLOWING MYSELF TO BE DISTRACTED; IF ONLY FOR A SHORT PERIOD OF TIME. THE ASSUMPTION ON MY PART; THAT THE STUDENT UNDERSTOOD 'FLY PRESENT HEADING' AND THAT HE WOULD COMPLY. SINCE THE STUDENT HAD PERFORMED MANY APCHS IN THE PAST AND HAD NO TROUBLE PREVIOUSLY DEALING WITH ATC; I DID NOT FEEL HESITANT TO PREPARE APCH PLATES WHILE HE SIMPLY HELD A PRESENT HEADING.