The flight instructor stated they performed a go-around because they did not know if the landing gear was extended. The aircraft climbed initially, rolled to the left and descended, striking the ground left wing tip and nose first. Examination of the runway showed slash marks caused by the propellers of the aircraft. Examination of the aircraft showed the landing gear handle was in the up position and the left propeller was curled over about two thirds of the blade length. The right propeller was curled over about one third of the blade length.