Champion Citabria flight Instructor reported inadvertent takeoff while practicing high speed taxi runs with student.

Narrative

During a training flight towards a tailwheel endorsement; permission was received for three high speed taxi runs on the runway. On each taxi run in which the pilot under instruction was in control of the aircraft during the run; control of the aircraft was turned over to the instructor pilot prior to the end of the high speed taxi run on the runway. During the final high speed taxi run on the runway; the pilot under instruction became uncomfortable and turned control of the aircraft over to the instructor pilot. During the transition of control of the aircraft the aircraft lifted off of the runway and it was determined that there was not enough runway to safely return to the runway before running out of runway. Before contacting the Tower to advise of the situation; the Tower inquired why a liftoff occurred when permission was only granted for a high speed taxi. Tower was informed that since the aircraft inadvertently lifted off during the high speed taxi evolution; it was requested to return for a full stop landing with a return to the ramp. In reviewing the scenario for tailwheel training; it was realized that it would be preferable to conduct this training at a non-towered airport or when at a towered airport only when 'cleared for the option' has been received. This will avoid an operation that is not specific to a tower clearance for safety of flight reasons.