The flight instructor reported that, during an instructional flight with the student pilot at the flight controls, they realized the airplane was high while they were on the base leg of the airport traffic pattern. After turning onto final approach, the student pilot conducted a forward slip to decrease altitude and intercept the glideslope. When over the runway, the student pilot had applied too much left rudder, and on touchdown the airplane immediately veered left. The student pilot then overcorrected to the right. The flight instructor stated, he then took control of the airplane and attempted to correct back to the left, however he was unable to regain directional control. The airplane departed the runway surface impacting a runway sign, resulting in substantial damage to the left wing. The flight instructor reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.