A C172 INSTRUCTOR AND STUDENT; UNABLE TO FULLY CLR RWY AFTER LNDG ACFT ON SHORT FINAL; LANDED ANYWAY.

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ON IFR TRAINING FLT FROM THV (YORK; PA) TO FDK (FREDERICK; MD) I WAS ADVISED TO CANCEL IFR ON GND AFTER VECTORS TO ILS RWY 23 FOR FINAL COURSE. WX WAS APPROX 2500 FT OVCST; BROKE OUT ON ILS/LOC AND SAW TFC IN PATTERN (3 ACFT) ROLLING ON RWY 8 WAITING TO DEPART. I CALLED MISSED APCH TO TFC AND ENTERED TFC PATTERN AT UPWIND. AT THIS POINT; SOMEONE INITIATED A TKOF ROLL ON THE RWY (RWY 23) THAT I WAS OVERFLYING. THIS ACFT WAS A CHEROKEE 6. SOON HE WAS CLBING UP JUST AHEAD OF US AND BEGAN TO TURN XWIND. I TOLD MY STUDENT TO TURN XWIND TO DOWNWIND. AT THIS POINT WE WERE ON DOWNWIND; THERE WAS AN ACFT ON FINAL; AND THE CHEROKEE 6 ON XWIND TO DOWNWIND TURN. ALTHOUGH AT THIS POINT A SUBSTANTIAL AMOUNT OF MY ATTN WAS DEVOTED TO TFC AVOIDANCE; WE CONTINUED A NORMAL TFC PATTERN AND LANDED ON RWY 23. THE CHEROKEE WAS RIGHT BEHIND US; SO I PLANNED TO DEPART THE ACTIVE AT THE FIRST TXWY AFTER DECELERATION; BUT ANOTHER ACFT WAS TAXIING ON THAT TXWY TOWARD OUR INTENDED TURNOFF FROM RWY. MEANWHILE; THE CHEROKEE 6 CALLED SHORT FINAL. I WAS VERY CONCERNED. THE TAXIING ACFT WOULD NOT GIVE WAY TO US TO DEPART THE ACTIVE RWY; AND THE CHEROKEE 6 SHOWED NO SIGN OF GOING AROUND AND I WAS IN BTWN. I WEDGED THE ACFT JUST OFF RWY SURFACE; BUT COULD NOT GET PAST HOLD SHORT LINE BECAUSE OF OTHER ACFT. THE CHEROKEE 6 LANDED; THE TAXIING ACFT TAXIED PAST AND I FOLLOWED. IN THE CONFUSION AND DISRUPTION; I FORGOT TO CLOSE IFR FLT PLAN.