I agree there are many areas of the education system which are perhaps not transparent enough from a parents' POV. But... Having every professional decision you make questioned can be very frustrating. And no I don't think teachers are beyond reproach!
The vast, vast majority of teachers want the very best for their pupils and will do everything in their powers to do so.
Having a class that range in ability from P scales up to level six is seriously tricky to manage and requires a lot of time and effort in planning. And can be very frustrating. I have a great girl in my y6 class who has a fixed number of 1:1 hours with a TA and gets them. She can only say a handful of words and her comprehension skills are very very low. Every day I'm acutely aware that she's not getting the best education with me, even though she's happy, because I and the TA can't give her precisely what's needed.
Slight rant there, but if you believe a teacher really isn't acting in the best interest of a child, SEN or not, I wouldn't let it lie. Because if I had that accusation thrown at me I'd have them there for bloody hours showing them all the stuff we are trying to do in difficult circumstances.