If anyone thinks any foreign company or group is going to be allowed to conduct a free, unbiased mass survey, poll or measure of the Chinese population, they are a fool.
For obvious political and power reasons, the Chinese Government will never allow such thing. If such a survey is allowed, it will be under their watchful eyes and “partnership.” Local officials, to avoid being embarrassed, will make sure the right schools and right students are involved.
So, you’re saying the Chinese collections of national test scores are guaranteed to be thoroughly cooked.
They certainly have the power to make that happen, and are not shy about wielding such power.