Devientenigma, I know where you are coming from. I alluded to the power of the institution v. the parent in another thread
DS (also SLD) is selectively mute at school. I have only just found this out but it has obviously been going on for a long time. The teacher has been complaining that he is very tired for some time and has consistently suggested - i.e. blamed us - that it is because we started him on Rispiridone and because we take him out sometimes in the evening (his passion in life is live music).
No matter how many times we have said that he is very verbal and very loud in all other settings, that he is on a sub-clinical dose of Rispiridone and that he goes out all of 8 times a year, the teachers continue to hint that we are to blame.
I have found that once a suggestion is made in an institution like an SLD school, it can gain momentum whether it is true or not. Any suggestion that it is something to do with them is not accepted. It can be the same with the NHS and it is very poor, non-reflective practice.
Great job, getting the Ed psych involved. I have finally got the SALT involved and the teachers are being persuaded to observe him in other settings and think about what else could be triggering his mutism, e.g., being scared of other pupils, being in a mainly non-verbal class, being bored etc. They have been shocked to find out how loud he is in his afterschool club.
Have you asked to go into his school and observe your ds in action? That is my next step.
Good luck, keep believing in yourself and keep a open mind on it.