Skylar
As soon as I saw the title of your thread I thought - I bet school won't write a supportive letter.
They can't when you think about it.
If they were to write a supportive letter saying your DS needed a Statement they would, in effect, be admitting that they had failed in their duty to apply for a Statement themselves if they considered one was required.
They couldn't admit that in a letter.
So they bang on (as you found) about anything except his school performance. They are also taking the 'blame the parent as he's OK when he's at school tack'. Very common.
I'm sorry this has surprised you. This is part of the really nasty way that we parents finally start realising that school are simply not playing fair.
Don;t send the letter from school with your Statement request.
The LA will write to school to ask how DS is getting on. The school will lie through their teeth to the LA so you might as well be prepared for that now. That's the reality.
It's a really vile feeling when you are lied to by professionals that you should be able to trust.
I would stop worrying about the application. Just stick your application letter into the post and wait for their response.