AN INSTRUCTIONAL FLT ENTERED SAN CLASS B AIRSPACE WITHOUT CLRNC.

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MY STUDENT AND I TOOK OFF FROM MONTGOMERY FIELD; SAN DIEGO; FOR A ROUTINE TRAINING FLT IN THE LCL AREAS. AFTER LEVELING OFF AT 1500 FT; I TOLD HIM TO STAY N OF THE MZB 084 DEG RADIAL TO AVOID CLASS B AIRSPACE. I ALSO TOLD HIM TO REMAIN OVER INTERSTATE 8 AT ALL TIMES TO AVOID FLYING INTO LINDBERGH'S CLASS B AIRSPACE. IN THE MEANTIME; THE ACFT'S R DOOR CAME OPEN. I TRIED TO CLOSE THE DOOR SEVERAL TIMES AND FINALLY GOT IT CLOSED AND LOCKED. AFTER TAKING CARE OF THE DOOR PROB; I NOTICED THAT MY STUDENT HAD FLOWN S OF THE MZB 084 DEG RADIAL. I IMMEDIATELY INSTRUCTED HIM TO TURN THE ACFT NBOUND. WE HEADED TO BROWN FIELD WHERE THE TWR ADVISED US TO CONTACT ATC FOR A POSSIBLE VIOLATION. I CALLED ATC RIGHT AWAY FROM MY CELLULAR PHONE AND EXPLAINED THE SIT THAT HAD TAKEN PLACE TO A PERSON NAMED MR. X AT LINDBERGH TWR. WE LEFT BROWN FIELD AND HEADED BACK TO MONTGOMERY FIELD (MYF) WHERE I ONCE AGAIN CALLED LINDBERGH TWR. I SPOKE WITH THE SUPVR AND EXPLAINED THE SIT IN DETAIL. HE STATED TO ME THAT EVERYTHING WAS OK AND THINGS LIKE THAT HAPPEN FROM TIME-TO-TIME. BOTTOM LINE: MY STUDENT INADVERTENTLY ENTERED THE CLASS B AIRSPACE WHILE I WAS DISTR WITH THE ACFT DOOR. I TRUSTED HIM TO STAY CLR OF CLASS B AIRSPACE; BUT HE PROBABLY GOT CONFUSED AND DIDN'T ASK ME ABOUT HIS POS. I LEARNED A GREAT LESSON FROM TODAY'S INCIDENT AND WILL BE MORE ALERT IN THE FUTURE. I APOLOGIZED TO ATC (SUPVR) AND TOLD HIM I WAS GOING TO EXPLAIN TO MY STUDENT WHAT HAD TAKEN PLACE TO AVOID A SIMILAR INCIDENT FROM OCCURRING IN THE FUTURE.