In traditional school, if your kid is ahead, the only move is to skip a grade. Now, they're the youngest in the class. This still doesn't solve the pacing problem or the fact that kids aren't uniformly advanced. A kid can be flying in math and behind in reading at the same time.
At Alpha, a fifth grader doing geometry sits right next to a friend who's catching up on fourth-grade math. Same room, same age, no one pulled out, no one held back. They're just each working on what they actually need.