JenG, I'd pursue a neuropsych evaluation outside of the school's evaluation, in parallel. They will find what they'll find, but they are not equipped to diagnose or treat-- they are looking only for educational implications of the behavior. Doing an outside evaluation helps to make sure that you get the full picture, and allows you to make sure the school is doing the right things for your child.
I agree with DeeDee on this. I also understand that neuropsych testing can be very expensive (we've been through it more than once). The one thing that I see as a potential downside to only going with school testing first other than what DeeDee has mentioned is that you may wait several months for the school process to unfold only to find you still don't have the answers *you* need for your ds and end up seeking out a private neuropsych eval anyway.
I'd try to do a few things right now while you're pursuing the school eval:
1) Definitely try to talk to an advocate to get a feeling for what happens in your area in situations like you are in - this may help you have an idea if your school really will be honest and helpful or try to prove that your ds is just fine and everything is rosy. They can also help you with a roadmap of what type of accommodations you can ask for or might want to ask for, what type of testing to be sure is included in the school's evaluation, etc.
2) Put everything that is happening down in writing if it's not been proposed in writing by the school. If you've had a meeting already where the school has said they will test, send an email to the team stating what you understand will happen, what is being tested, and what the time frame you expect it to take will be (if they haven't given you a time frame). You shouldn't make up that time frame from anywhere, there should be a time frame that your state requires the testing to be completed in (our state law is, I think, 60 days, and our school district policy states 45 days). If you're not aware of that deadline there is a chance the school will not rush to get anything done. Your written statement will trigger either the start of that time frame or a response from the school that you'll start with an eligibility meeting (which would then in turn start the timeframe) - I am SO totally not explaining this very well.. which is why you need to talk to an advocate lol.
3) Familiarize yourself with your rights in your school district. If the school does their evaluation and you don't agree with the findings, you should be able to request an Independent Educational Evaluation outside of the school - but you will still most likely have to choose from a certain set of evaluators which might not include who you would have chosen if you'd gone the route of a private eval yourself... so it's somewhat helpful to think through that possibility ahead of time.
4) Have you checked into whether or not your insurance would cover a private eval? As DeeDee mentioned, the school will not diagnose or treat, and if there is something going on, this is in many ways more key than simply having accommodations and a plan at school. School is just one part of our kids' lives - we need to understand what's behind behaviors at school to help our kids navigate their lives. I don't know anything about your ds, but it's possible he might need counseling, and if that was the case, the school isn't going to recommend the type, or who to go to, or even tell you it would help. If he would benefit from OT, but doesn't fall under the extremely low bar for needs set by most school districts, the school isn't going to tell you about it, they'll see your ds as "fine". Our 2e ds didn't meet the low bars for either OT or SLP set by our school district, but OT helped him quite a bit early on and he's been working with an SLP for several years and that honestly has been the one thing that has enabled him, with his specific set of challenges, navigate school and life - it's honestly been a life-changer for him - but we never would have even known to try it out if we'd relied on his school eval.
Lastly, the reason I'd pursue learning more about your private neuropsych options now is time - the school eval will take a bit of time, but it's also likely that getting an appointment with a neuropsych will also take time, and if you got to the end of your school eval and suddenly realized that you are still wanting more in terms of understanding what's up, you might not want to have to wait another 3 months again for the neuropsych appointment.
Best wishes,
polarbear