I'd be following up in writing, with a list of even higher ups listed at the bottom of the letter, with "cc to..." in front of their names. Then follow through with getting copies to these people. I would include in your letter the immortal words..."the urgent assessment as requested in writing on OCt 23 2006, which I have been told is to be carried out by an intern - please explain the rationale behind giving such an important task to an unsupervised and not fully qualified student. Also please give me reasons, in writing, for the delays to this date."
thank you is your six year old? What proportion of his life has he been waiting for this "urgent" assessment? I would also include this calculation. When did you fist request this verbally? I would make reference to anything like this, use the second letter to document any history of neglect by the system and ask for written response to be expedited, so that you can make alternative arrangements should they become necessary.
Be polite, unemotional, but clearly annoyed. Try to not annoy the higher-ups you're sending the letter to - remember that you want action, don't use it as a chance to vent. Just document the problems and ask for an explanation as well as some action to make up for the delays to this point. Add the argument that assessment and intervention has got to be of major benefit to the education system, in providing accurate information on how best to manage this child without causing too many difficulties in the classroom. Sell it to them, make it clear that you are reasonable. It makes any lack of response seem unreasonable, which is what you want.
Good luck. Kick a*se.
Marg