Why is a child penalized in some cases when he/she transfers from a public school to a private school?

Well, it sounds like you are asking for a test, but saying that if they don't p the test they repeat the grade. I went from Public to Private schools in 5th grade. I was so far ahead in public schools. Never had homework, was bored out of my mind, and always ended up getting extra curriculum to keep me busy. The reason they may make you repeat is because to keep up with their level the school requires, the child may need to repeat the grade. Its like if all freshman at the private school are doing Calculus, and the highest he had at public schools is Algebra I, they prob won't grasp the concepts. School is all about building blocks, and unless you have a certain foundation you will not make it to the next level. It is unfortunate that it isn't easier to make the transition, but that's why they are "better" schools. They teach more advanced material.