[bayesian_horse]

The assumption there is that the ordering of the students by performance is stationary, particularly in the first few years. Regardless of personal situation.

If a child catches a flu at the wrong moment, it will get stuck way below its level for all its career.

Khanacademy data has shown that students in a math class will progress at different speeds at different times and that their "ranking" can invert multiple times over a school year or beyond. It's not exactly the same, but their system makes sure students don't stay stuck.