Can not believe how useless sons school have been - how to complain?

Son is 15 now. Hes had "issues" for around two years but only got formally diagnosed last october (I guess we all know what CAMHS are like). CAMHS at the time sent the letter to school also.

We've had various conversations over the years with the school with various head of years etc. For instance, he has a bit of a toilet thing so they let him use disabled toilets. They did also sort out counselling with a person that came to the school.

Fast forward to last week. School parents evening. NONE of his teachers knew he had been diagnosed.

Had a chat with the Special Needs teacher. Sat next to her was a lady who I remembered. So I mentioned son to her - never heard of him, not aware. Her assistant had and had arranged the counselling sessions (but obviously had not told her colleague). I asked why they hadn't informed his teachers - not seen the letter.

Turns out the letter arrived at the school last October and has only now been seen by the Special Needs lady. Nice one. Even then only because I mentioned it at parents evening.

She phoned today and was really rude. Not her fault apparently and, anyway, we should be grateful that a meeting has now been sorted in MAY between CAMHS and the school. This should all have happened 18 months ago!

Surely a Special Needs teachers job is to read letters from CAMHS about the pupils? Otherwise I can't see the point.

They have been useless. Whats the best way to complain? To the governors? MP? Council?