C172 FLIGHT INSTRUCTOR AND STUDENT EXPERIENCE RUNWAY EXCURSION AFTER LANDING WHEN STUDENT FREEZES ON CONTROLS AND INSTRUCTOR IS UNABLE TO OVERPOWER HER.

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I WAS FLYING WITH A STUDENT. AFTER RETURNING FROM THE PRACTICE AREA; WE REMAINED IN THE PATTERN TO WORK ON LNDGS. THIS WAS THE STUDENT'S FIRST TIME WORKING ON LNDGS -- THE FIRST FEW WERE OK; HOWEVER; WE ENCOUNTERED A PROB AFTER OUR LAST LNDG (WHICH WAS FULL STOP). MY STUDENT HAD THE CTLS AND I TOLD HER I WOULD TAKE THEM OVER ONLY IF I DEEMED NECESSARY. THE TFC PATTERN AND DSCNT TO LNDG WERE VERY GOOD; WINDS WERE RELATIVELY CALM; AND THE FLARE TO LNDG AND THE LNDG ITSELF WERE ALSO VERY GOOD. WE LANDED ON THE CTRLINE AND IMMEDIATELY AFTER TOUCHDOWN MY STUDENT FREAKED OUT AND HER REACTION WAS TO SLAM ON THE BRAKES. SHE SLAMMED ON THE BRAKES WITH THE BALLS OF HER FEET; THE AIRPLANE WAS VERY DIFFICULT TO GET UNDER CTL. I YELLED AT HER TO GET OFF THE CTLS AND TOLD HER IT WAS MY AIRPLANE. I WAS UNABLE TO OVERPOWER HER WHILE SHE HAD THE CTLS AND DID NOT HAVE COMPLETE CTL OF THE AIRPLANE IN TIME TO PREVENT US FROM GOING OFF THE RWY. HER REACTION AFTER TOUCHDOWN CAUSED US TO GO OFF THE RWY INTO THE GRASS FOR A FEW MOMENTS. I WAS ABLE TO REGAIN CTL OF THE AIRPLANE IN TIME TO TAXI US BACK ONTO THE RWY. TWR DID NOT SAY ANYTHING; SO I ASSUME THEY DID NOT SEE IT. WE WERE ONLY OFF THE RWY FOR A SHORT TIME; BUT IT WAS ENOUGH TO SCARE THE TWO OF US. I HAVE NEVER HAD A STUDENT WANT TO APPLY THE BRAKES SO RAPIDLY AND FORCEFULLY AFTER TOUCHDOWN. I HAD ALREADY EXPLAINED TO HER A NUMBER OF TIMES HOW IMPORTANT IT WAS TO BE EASY ON THE CTLS; ESPECIALLY THE BRAKES; BUT WHEN FEAR KICKED IN SHE REACTED DIFFERENTLY. KNOWING HOW TO DEAL WITH STUDENTS WHEN THEY FREEZE UP ON THE CTLS AND PLACING MORE EMPHASIS ON WHAT TO DO AFTER TOUCHDOWN CAN HELP PREVENT OCCURRENCES LIKE THIS IN THE FUTURE.