DS had a neuoropsych eval at a well-respected place and the school is ignoring all the recommendations, saying that a medical disability is different than an educational disability (he was diagnosed with developmental coordination disorder among other things). I would not trust the school a bit to do a decent eval, but on the other hand if they ignore or dismiss outside evals, what good is that either.
DS already has an IEP but they won't add any services to it unless they do their own comprehensive eval (which will probably be bogus).
It definitely doesn't hurt to get an outside eval. It helps you to convince the school of what he needs, and it is good for your own information, but the school isn't forced to accept it.