Hi A.,
I liked Moe's response, being that she was a speech therapist in the school system. I too have my son in speech therapy at his future elementary school. He is the same age as your son, unfortunatley we don't qualify for Pre-K so I am preparing him and my 3yr. old for school myself.
When your son was first tested, did you attend an ARD, which stands for Assist, Review and Dismissmal, which is the plan that the school and you the parent agree to which covers his needs and the goals to achieve progress? I honestly don't think that his teacher being in communication with his therapist was wrong, but I too would question the therapist and the school communicating with his pediatrician without your knowledge...may not be wrong, but as a parent I too would expect to be informed on any area concerning my child.
I recommend you call the special education services dept. of the school district that your son attends and also the principal of the school. Believe me, when the principal hears that you called the district offices, any question you have will be addressed immediately.
Please take Moe's advice about your approach...the school system is so used to dealing with either irrate parents or parents who just don't give a damn. If you present yourself as a rational, intelligent person who sincerely cares about her child, they are going to deal with you the same way...just remember, there are laws and standards and if any of these are not followed, the school can get in a lot of trouble.
Good luck with this,
J.