See also the recent discussion on same-age vs mixed-age classrooms. By mixing age groups, the kid who is advanced in history can hang with a grade or two above her own. But she might be a grade below in writing, and that should be 100% ok.
I was a poor kid that attended a gifted program in first grade, the majority of the parents were upper middle class doctors and lawyers. It was lord of the flies classist bs. I clearly wasn't supposed to have been let in and went back to normie classes. It wasn't again until 6th grade that I was back into a one class a day advanced program.
I don't think there should be separate classes for advanced or gifted students. Many of the aspects of gifted programs are just better funded less shitty versions of regular classes. Just like the startup within a corp trope, the gifted programs are often just cover for a private school within a public school.