The student pilot landed long on an intended touch and go landing and therefore, rolled out to a full stop on runway 31. A 180 degree turn was made at the end in order to take off on runway 13. Prior to initiating the takeoff, according to the student pilot, he placed the fuel selector in the right tank position since it was the fullest of the two tanks. Very shortly after takeoff the engine lost power. The CFI instructed the student in the left seat to switch tanks, which he did. With the CFI taking control of the aircraft, a forced landing was made in a field rolling through a barbed wire fence and collapsing the nose gear. Neither pilot changed the position of the fuel selector after the accident. It was found in the left 'off' position. The fuel selector had four positions: left and right at the 10 o'clock and 2 o'clock positions, and two 'off' at the 4 o'clock and 8 o'clock. Either off setting can be selected from any other position. The right tank was full of fuel and the left contained 1 gallon. No fuel remained in the engine driven fuel pump output line.