> The problem is making someone repeat a grade destroys their social life leading to even worse grades later on

I am from Germany where repeating a grade is quite common and uhm what? How does it destroy their social life?

It isn't even as bad as changing school as you can still see your friends. I don't remember anyone being traumatized or ostracized by having to repeat a grade. Also some high performing students are also allowed to skip grades and no ones says that it ruins their social life.

Sometimes students just need an extra year to mature.