I think somewhere we got away from teaching the student and started teaching the curriculum. Things have become so standardized that as long as you pass the test, you are considered "taught".
Differentiation requires the ability to accurately assess where a child is functioning. In school, this can be extremely difficult! Especially if the tests don't provide this kind of information or the teachers aren't trained to interpret the tests.
One more thought, in my state, there is accelerated education with compacting and faster pace. It's used to bring the lower functioning kids up to grade level. So, if you google acceleration in my state, you will find programs for below grade level kids.
Yes! They just want to stamp them "PASSED." When I taught honors/AP English I was on my own for actually teaching the gifted kids something new.
