According to the accident report form submitted by the flight instructor, the student had lined the glider up for landing, and a prelanding assessment indicated that the winds were calm. The purpose of the flight was to practice slips to a landing with no spoilers. While on short final, the slip was reduced to maintain the glide path, at which time, the glider encountered a sink rate. The student maintained the glide path; however, while in the flare over a fence, the left wing impacted a fence pole, resulting in substantial damage to the glider. The flight instructor stated that there was nothing mechanically wrong with the glider prior to the accident.