A guard fence system is useful for preventing persons from falling off bleachers. The guard fence system comprises at least two upright posts mounted to the bleachers. Each post has a lower U-channel and an upper U-channel spaced a predetermined distance apart. The lower and upper U-channels have respective slots that open into the top ends of the U-channels. A guard fence member comprises a rectangular frame covered with a grill material. Upper and lower pins extend oppositely from the frame vertical members. The lower pins of the guard fence member are inserted into the slots of the lower U-channel. The guard fence member is pivoted to a generally vertical attitude such that the guard fence member upper pins are proximate the upper U-channels on the posts. The guard fence member is lifted and further pivoted until the upper pins thereof are vertically over the slots of the corresponding upper U-channels on the posts. Then the guard fence member is dropped so that the upper pins thereon enter the slots of the upper U-channels. The slots on the lower U-channels are sufficiently long to prevent the guard fence member lower pins from disengaging from the lower U-channels when the guard fence member is lifted. A bolt is removably inserted into at least one of the lower or upper U-channels to prevent unintentional lifting of the guard fence member out of the slots.