RPTR OFF ASSIGNED ALT AS PER WRONG ALTIMETER SETTING.

Narrative

WE LEFT LCI HEADED FOR IPT. WX WAS GOOD WITH CLOUDS FORECAST TO BE 3000-4000 FT SCATTERED; BROKEN. IPT (WILLIAMSPORT; PA) WAS RPT CLR. THE CLOUDS DID FORM NEAR THE MOUNTAIN; AT 6500 FT; VFR WAS DIFFICULT TO MAINTAIN. WE CLBED TO 8500 FT. SOON THE CLOUDS BEGAN TO GET CLOSE EVEN AT THAT ALT; SO I ASKED FOR AND GOT IFR CLRNC TO WILLIAMSPORT DIRECT AT 9000 FT. SHORTLY AFTER RECEIVING THE CLRNC THE CTLR CALLED US AND ASKED US OUR ALT. I WAS NOT QUITE 9100 FT SO I RPTED 9000 FT. HE RESPONDED BY SAYING HE SHOWED US 300 FT HIGH; AND GAVE US THE ALTIMETER SETTING AGAIN. TO MY SHOCK IT WAS OFF OVER 100 FT. FLYING FROM THE R SEAT THE ALTIMETER ADJUSTMENT IS COMPLETELY BLOCKED. 1 OF 3 THINGS HAPPENED: 1) THE STUDENT INCORRECTLY SET THE ALTIMETER IN THE FIRST PLACE. (THE STUDENT HAS ABOUT 350 HRS ON THAT AIRPLANE AND ABOUT 500 TOTAL); 2) HE SET IT CORRECT; BUT DEVELOPED A BAD HABIT OF CHANGING IT WHEN HE HEARS A NEW SETTING. HE MAY HAVE REACHED OVER AND CHANGED IT WHEN I WAS SCANNING FOR TFC. POSSIBILITY 3) IS THAT THE XPONDER/MODE C HAD JUST BEEN WORKED ON AND WAS BONKERS FOR JUST A FEW MINS. IT GAVE US TROUBLE THE REST OF THE TRIP BY OCCASIONALLY DISAPPEARING FROM THE SCOPE. THIS WAS A STUDENT THAT WAS VERY NEW TO ME AND AN ACFT THAT WAS JUST AS NEW. I WAS NOT AWARE THAT THE KOLLSMAN WINDOW WAS INVISIBLE FROM THE R SEAT. EVEN LEANING WAY OVER DID NOT HELP. THE STUDENT WAS TOO NEW TO ME TO HAVE AN EDUCATED GUESS AS TO PREDICT HIS REACTIONS. I CAN PROMISE I WILL BE MORE CAREFUL IN THE FUTURE.