I think that the problem with your proposition is that often, HG kids would be bored and staring out the window and the teacher doesn't think of them as gifted. And MG kids tend to be identified by teachers. If you are talking about some type of yearly testing, that is expensive. I think in most high schools, if a student is motivated, they can accelerate. I had skipped in grade school, when they had standard skipping in grade 4. And in high school I could choose my subjects and skip another year. In a regular high school. In grade school, that cannot happen with a lack of resources that most schools experience. You need better school systems in poverty areas, to enhance opportunities. And covid has really destroyed some kids education in those areas, where parents don't have resources to help them. Very sad.
