I'm sure this varies from school to school, district to district, and state to state. Where I live, if a parent opts out of state testing, the child's exam is given a zero and that score is counted. Since it's counted, it makes the district cranky when parents do this.
I'm guessing that parents who do this are labeled by the district as "problem" parents, and because of this, the district may be less than willing to accommodate other special requests (such as acceleration or differentiation) than it might otherwise be.
I do know that in our district state testing scores (along with other criteria) are used to decide who gets to take Algebra I in 7th grade. It would not surprise me if they used the scores for other placement decisions as well.
