Hi Marion, I'm Connie, the ADD editor and I currently teach in a special day school. I'm sure that other readers also have information.
Here are things that I look for:
Does the child feel ready to go outside of the special school?
Does the child WANT to go? (Some do not!)
Is the child able to manage frustration?
Can the child wait for help, or does help need to be immediate?
What has the child's classroom behavior been like over the past three months?
If the child has mood cycles, where are they in the cycle?
How are the child's academics?
Can the child advocate appropriately for himself?
Is the parent ready to advocate for the child?
This is by no means a total list, but it is something to think about.
If the child has ADD as well as other special education needs, there is other school-related information on the ADD site.