A Long Board (長い板きれ Nagai Itakire ) is an item that appears in CORPSE-PARTY and CORPSE PARTY -Rebuilt-. It is a long wooden plank that can be used to bridge longer gaps in the floor that would otherwise be uncrossable.
The first long board can be found in the second floor north wing, right outside Classroom 2-9, you use it to bridge across a long gap leading to the incinerator and the two classrooms.
The other one is found inside classroom 1-1, however, if you leave to the hallway while carrying it, it breaks. You can then find it again the the same spot, you use it in classroom 1-2 to bridge your way to the cabinet.