I asked the teacher next to me why they don't have bleachers.
Teacher: What are bleachers?
Me: <draws a picture>
Teacher: That takes up a lot of room.
Me: They fold up against the wall when you're not using them.
Teacher: I've never heard of such a thing.
So there you have it. Japanese schools don't use bleachers because they don't know about them. They don't play spectator sports in the high school gym, so they don't ever need them for games. They go elsewhere for that.
The "pep rally" was very...Japanese. The students on the sports teams put on their uniforms and then paraded up to the stage, where the vice-principal gave them a pep-talk and each team gave a short speech. The rest of the students sat on the gym floor and listened (or napped).