I'm not sure that's true. In Japanese pre-adult manga, the characters are usually a little older than the intended audience. The idea is that the kids want to read about the people they hope to soon become, not the people they are now. And maybe it helps the readers to see characters going through the challenges they anticipate they will face. So, a"middle grade" novel could conceivably have a main character who is just entering high school, but not a high school senior. A senior could, I imagine, be an antagonist; older kids often are, and have enormous advantages. However, I'm speaking way outside my zone of expertise. Hopefully someone who knows better will comment.