Good letter.
And sometimes just a casual mention of FERPA in a phone call will also do the trick, especially if you don't really care about seeing all of the rest of the file. I believe however, that it only applies to public schools, and I wasn't sure if you were in private or public.
Principal: "Oh no, we can't provide any additional information, the school psychologist's report is kept confidential."
You: "Oh, maybe I misunderstood, but I was almost positive that under the Family Educational Rights and Privacy Act all testing information had to be shared with me. Maybe you want to check with the district on that, or I can check with my brother in law who does education law in Poughkeepsie..."
Cat