Honestly, a teacher that can't at least do that either has her hands tied administratively, or just doesn't care,

.
I think there may be an administrative issue when it comes to "differentiation" here. Here is the stated district policy.
All teachers of GATE students will follow the core curriculum for the grade level and
provide an enriched and differentiated program for the GATE students throughout
the school day.
So even if the teachers want to provide differentiated material, they have to follow the core curriculum first. At least, this seems to be how they interpret this.