Having parented both exceptional children, "regular" children and special needs children, I can say with absolute certainty that most curriculum-based measurements do not work for either exceptional children or special needs children. The differentiations that are noted are not those skills that are most important for either the exceptional or the special needs child. The most important measurement that should be taken should be social skills, which is difficult to address in a school setting.
Basic comprehension and mental acquisition is what curriculum based measurements evaluate. Has the student met the expectations of that level? If the answer is yes, then nothing further is done. If the answer is no, then new action must be taken that targets the inadequacies. There is no curriculum based response for children with special skills.
I think its hard to find every special needs child,but they can find some. The budget just doesn't allow it. Parents are also to blame for some of this, they refuse to see that their child needs some kind of care that is outside the realm of normal, it is up to them as well.
Schools, public schools in particular, are ill equipped and not prepared to deal with children with special skills. The best that we can hope for now is that they test out of the education system as a whole and get into programs that typically are not meant for them at their age.