Long ago I helped coach some kids' baseball teams. There I got a notion why 60% is or was the minimum passing grade in schools: if you give a kid a task he can perform about 2/3 of the time, it may engage him and he may get better at it. If you give a kid he can perform half or less of the time, you will get a frustrated, then bored and put-upon kid.
Moving a kid who has not mastered basic material into a class that aims to build on it? That seems to me a formula for frustration.