I think it will be an interesting experiment, we shall see if/how different the classes end up being. In NYC they don’t seem to be able to pull off the regular high school admissions test due to covid restrictions, and there is a movement to just use lottery instead. We’ll see where that ends up soon.
I have to say that IME the kids struggling at boarding school did/do not struggle due to low SSAT but due to inadequate time management skills (much like @cinnamon1212 suggested). My own high 90s SSAT kid had a hard time with it, the official study hours end much earlier than he was used to doing his work, and are not sufficient to get everything done. It took him time to learn to use his free periods and Sundays to get a head start on the week. I know some kids needed help from learning center to do this, or even executive functioning tutor. But I don"t think it has much to do with intelligence and academic ability. Plus there is the willingness to grind, some kids strive for all As/A+ grades from most challenging classes and are willing to put in the work it takes to get there. A lot of kids are not quite so motivated, or their motivation is toward their interests only, academic or otherwise. In order for class to work well you need a balanced class with kids in all the categories.