What I've done in the past is to be prepared with a portfolio of work that included dated material (we started dating DS's writings when we thought that he might be a bit advanced). I didn't always use it, but it was there in case we needed it.
I'd also print out articles that I thought were relevant to the meeting. In our case we were advocating for early admissions (essentially a skip of K) so I was prepared with articles about how socialization is not as big of a concern as most people think.
And finally, we did have a psychology's report that we obtained privately. But our situation was a little different.
So bring with you things you've tucked away. Depending on where the conversation goes, you can decided to show your DS's work. A lot of it depends on what you want to get out of the meeting. Are you just trying to get the school to acknowledge that he's gifted, or are you advocating for something else (subject skip, etc.).
Good luck.
JB
