KD, you will want to pick up the Wrightslaw book From Emotions to Advocacy; it's a primer on the process with sample letters for corresponding with the school. Their website is a little chaotic but also full of good information. You will need to know the process and your son's rights.
The school can take an outside diagnosis into account, but must by law do their own evaluation of how the disability affects your child's education. They are looking for educational needs. You can start this process by writing a letter requesting an evaluation and listing all the things about your child that you think affect his participation in the educational setting. Include social issues as well as academic issues. Describe the settings that cause the most trouble for your child.
They then have some set number of school days to complete their eval (I think 60 school days, which is a long time). This may include classroom observations, psychological testing, IQ testing, OT, PT, speech, whatever they think they need.
Then you have a meeting to discuss their findings and they make recommendations for services. I strongly recommend bringing an outside professional you trust to this meeting to act as your DS's advocate; it can be very overwhelming to be in one of these meetings surrounded by school professionals who are telling you things about your kid.
It is true that services are driven by needs; but it is also true that some schools tend to overlook kids with diagnoses they deem as "no big deal." If you start to think the school is not doing your son justice in determining his services, I'd recommend hiring an educational advocate (there are some who serve 2E kids) to work on your son's behalf. (Some states have a free Legal Rights Service for people with disabilities, too.) A good advocate has expertise a parent just doesn't, and can get you through the steep learning curve phase.
Good luck,
DeeDee
