The best resource available is: you.
You have observed your DS in a variety of environments. Some of them (the church-related ones) offer a direct parallel with school. How is that going? What kind of behavior do you see? How do your observations compare to what your teacher reports?
Depending on that, your counter-argument might be something like this:
"We do not see the kind of behavior you describe in similar situations. The problem is specific to school. We believe this is due to lack of challenge and engagement, which leads to boredom, and acting out. The GT pull-out should be beneficial in addressing this problem."
Or:
"Yes, we see DS behave this way in certain, similar situations, generally due to boredom. We know that he's not being challenged with his current curriculum, which explains why he's experiencing boredom in your class. The GT pull-out should be beneficial in addressing this problem."
Depending on whether your state offers any guarantees to gifted children via legislation, you could also add:
"My DS has a right to an education that is appropriate for his cognitive abilities. We do not accept any perceptions of 'immaturity' as sufficient reason to deny him access to that."
