NMAC ON SHORT FINAL AT NON TWR ARPT UNICOM.

Narrative

I WAS ON AN INST TRAINING FLT. ON BOARD; THERE WERE TWO STUDENTS AND MYSELF AS THE INSTRUCTOR. ONE STUDENT WAS UNDER THE HOOD BEING VECTORED BY ME. THE OTHER STUDENT WAS IN THE BACK SEAT WITH INSTRUCTIONS TO HELP WATCH FOR TFC. WE ENTERED A R PATTERN FOR 24R ON A 45 DEG ENTRY; 1000 FT AGL; THEN ENTERED DOWNWIND; BASE AND THEN TO FINAL. THE HOOD WAS REMOVED WHEN ESTABLISHED ON FINAL. A POS RPT WAS MADE ON EACH LEG OF THE PATTERN 45 DEG DOWNWIND; BASE AND FINAL. ON SHORT FINAL; MY STUDENT IN THE BACK YELLED MY NAME. I GLANCED OVER MY L SHOULDER TO SEE WHAT WAS WRONG. THE STUDENT IN THE BACK WAS POINTING OUT THE R WINDOW. I SPUN AROUND TO LOOK OUT MY R WINDOW; WHILE DOING SO; I LIFTED THE R WING TO SEE. THERE WAS AN ACFT JUST ABOVE AND SLIGHTLY BEHIND US. AT THIS TIME WE WERE ON VERY SHORT FINAL AND ABOUT 250-300 FT AGL. I MADE AN EVASIVE ACTION BY ADDING FULL PWR DIVING DOWN AND BANKING TO MY L; TURNING 180 DEG. I STAYED LOW TO THE GND UNTIL I WAS SURE I WAS OUT OF THE OTHER ACFT'S WAY. I THEN CLBED OUT; CIRCLED THE ARPT AND REENTERED THE PATTERN ON A 45 DEG ENTRY.