Much of a child's educational performance is determined by factors outside of school, so measuring teacher or school performance using tests is "noisy." However, despite the difficulty of measuring student performance, each parent has a signal of whether it is working for their own kids. This is what makes vouchers so attractive as a scheme to better align the incentives of schools with the goals of parents. If parents can vote with their feet, and if money follows the parents, then schools have an incentive to do better.
Even if only a fraction of parents get a good signal, the resulting market pressure may be enough to cause improvement.