Why is it the goal to help every student reach their potential? Who decided this, and what authority did they have to decide for us what the goal of all schools everywhere automatically must be?

Also, you absolutely can't 'let' kids move at different speeds in the same school. You can let kids move at speeds where you enough kids at that speed. For a 90th percentile kid that is fine, there are roughly 10% of the students that share their speed (or nearly do) so you can form a class of them. For the top 1% or top .1% you just can't, and the 99.9th percentile kid can learn much, much, much faster than the 90th percentile kid, and can immediately understand concepts that the 90th percentile kid won't be able to grasp until they are years older (if ever). Putting those kids in the same class means one will be impossibly behind or the other will be incredibly bored.