Hi! I grew up on my LI and my brother still lives there; I have many friends who also live there and they are spread out all over the place.
The Smithtown School District is good, diverse district and there is money there, so they do have services. My brother had to get services for his oldest daughter years ago, about 10, and he said it was difficult. High taxes!
In the Rocky Point/Sound Beach area, the district is also good. It's mostly middle to upper middle class so there is also money there and they have services, though my friends who live there have never had to take advantage of them or fight for them. Moderate taxes.
The Three Village School district, which encompasses Port Jefferson, Setauket & Stony Brook towns, has lots of money and I have a friend who has very up close and personal experience working with the district to get her son services and she said they were awesome. This is a diverse district, meaning it's not all white, all black, all poor, all rich - but it is a wealthy area and the schools have had a very long rep for having EVERYTHING. The taxes are through the roof!
The Shoreham/Wading River areas are very nice, a little less city and a little more country; there are pockets of wealth that has boomed over the past 15 years or so. I have friends who live there with their four sons, two of which needed services and they did okay. I don't know the extent of their son's needs, but I beleive their needs are being met without a lot of battling.
Growing up there, no one ever had much faith in any of the schools that ran along the south shore of LI, unless they were out in the Hamptons. I grew up in Commack, which is a part of Smithtown and back in my days, their schools were in the top ten of the nation. However, being a difficult child in HS, I can tell you that they were not very helpful - but, that was back in the late 70's. Things and laws have changed.
I believe that it's perfectly okay to research districts and find out how they rate across the state/country. I have a friend who is a realtor and when she shows a home she always has the data on the district, etc. You could also contact the superintendent and discuss your difficult child's needs and how their services would be able to help, etc.
Best of luck. I miss LI, but not the costs. I mostly miss the water.