I think these letters are the hardest things in the world to write.
If it's okay, I'll put on my writing tutor hat and ask you some questions...
What is it that you most want to convey to the teacher? What is the most important point for you to make in this letter? What is most important for the teacher to know about your DS?
Then, are there any secondary points you want to make?
Next I think you want to consider if your secondary points are going to compete with your primary point. If they will, then you either have to carefully craft your letter to include them--and that poses a risk to your primary message--or you have to leave them out and deal with them in future interactions with the teacher instead.
It sounds like how you present yourself in the letter is an issue for you, too. You don't want her to think you're one of "those" moms who push their kids, right? I think that's a tone issue more than anything.
Do you want to discuss the skip in the letter? Or do you want to stay focused on your DS's needs? That might help with how you're perceived. If she thinks you're too focused on the skip, she might get the wrong idea about you. But then again, you don't want to ignore the elephant in the room...
I don't know if this helps...
