INSTRUMENT INSTRUCTOR HAS ALT DEV ON IAP WHEN FORCED TO TAKE OVER FROM DISORIENTED STUDENT.

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I WAS WITH A STUDENT ON AN IFR TRAINING/BUSINESS FLT. HE IS AN INST STUDENT TRANSITIONING TO GLASS COCKPIT FLYING. WE WERE ABOUT 2 HRS INTO THE FLT AND HE HAD BEEN FLYING THE ENTIRE TIME. THE WX AT EKX; OUR DEST; WAS IFR. ATC CLRED US FOR THE VOR-A APCH TO EKX AND HE BECAME CONFUSED IN TRYING TO SET UP THE APCH. I THEN LOADED AND ACTIVATED THE APCH AND IT BECAME OBVIOUS; AT THE LAST MIN; THAT I WOULD HAVE TO FLY THE APCH. WITHOUT PROPER TIME TO BRIEF IT MYSELF; I CROSSED THE NEW HOPE VOR AT 2500 FT MSL AND; RATHER THAN MAINTAINING 2500 FT TO SYNDI; THE FAF; THEN DSNDING TO 1700 FT TO MAYHO; I MISREAD THE APCH PLATE AND DSNDED TO 1700 FT BTWN NEW HOPE VOR AND SYNDI. WITHOUT PROPER TIME TO PREPARE; AND JUMPING IN COLD; I SHOULD HAVE ASKED ATC TO BE VECTORED AROUND FOR ANOTHER ATTEMPT. EVEN THOUGH THE STUDENT HAD DONE EXTREMELY WELL UP TO THIS POINT; AS IN PREVIOUS LESSONS; I SHOULD NOT HAVE ALLOWED MYSELF TO GET COMPLACENT AND SHOULD HAVE FULLY BRIEFED THE APCH AS IF I WOULD BE FLYING IT. LESSON LEARNED.