The invention discloses a counter balance for teaching of the middle school. The counter balance comprises a pedestal, a middle knife rest, a lever with scales and pallets, the pallets are mounted atthe two ends of the lever via hangers respectively, vertical support arms are arranged in the lower ends of the hangers respectively, jacks are formed in the upper end surface of the pedestal, the lower ends of the vertical support arms extend into the jacks respectively, the bottom of each vertical support arm is provided with a first magnet, rotating discs are arranged under the jacks respectively, multiple protruded teeth are formed in the excircle sidewall of each rotating disc, second and third magnet are arranged on the protruded teeth at interval, the end, close to the first magnet, ofeach second magnet and the first magnet are heteropolar, the end, close to the first magnet, of each third magnet and the first magnet are homopolar, a through hole is formed in the bottom of each jack, the second and third magnets can be inserted into the through hole, a connecting groove communicated with the rotating disc is formed in the bottom of the jack at one side of the through hole, a limiting plate is inserted into the connecting groove, a spring sleeves the limiting plate, and the two ends of the spring are fixed to the inner wall and limiting plate of the connecting groove respectively. A knife edge can be protected effectively when the counter balance is disposed.