THE CFI STATED THAT DRG TKOF, THE ACFT ACCELERATED SLOWER THAN NORMAL. AFTER ROLLING ABT 1800', THE STUDENT ROTATED THE ACFT TO A NORMAL CLIMB ATTITUDE & IT CLIMBED APRX 5' TO 10'. AT ABOUT THAT TIME, THE CFI NOTED THAT THE AIRSPEED INDCN HAD DROPPED TO ABT 40 KTS & THE ACFT BEGAN TO DSCND. HE THEN TOOK THE CONTROLS & ABORTED THE TKOF WITH ABOUT 700' TO 800' OF RWY REMAINING. HE APPLIED BRAKES, BUT THE ACFT SKIDDED OFF THE RGT SIDE OF THE RWY AT THE DEP END & HIT A CONCRETE PAD CONTAINING RWY END/THRESHOLD LIGHTS. THE TEMP WAS 90 DEG & THE DENSITY ALT WAS APRX 2250'. THE CFI EXPECTEDA LONGER THAN NORMAL TKOF ROLL, BUT AT THE TIME OF THE OCCURRENCE, HE COULD NOT UNDERSTAND WHY THE INDCD AIRSPEED HAD DROPPED. RPRTDLY, THE OWNER HAD INSTALLED A MOVABLE COVER ON THE PITOT TUBE TO KEEP INSECTS OUT, WHICH COULD BLOCK ITS OPENING DRG HI ANGLE OF ATTACK OPNS.