Renzulli's Compacting Curriculum program showed that teachers could identify targeted students and eliminate half the curriculum for them without impacting their achievement on tests aimed for students a year older. The teachers usually provided enrichment in that freed-up time, and sometimes accelerate. I've said before in this forum, I'd like to see an array of supports and services for different types and different levels of gifted and creative students, the way we have a wide variety of supports and services for different types and levels of disabilities.
The growth NCLB requires is growth in the percentage of students who score as proficient on their state tests. This is an alternative to meeting specific targets of proficiency for the student body as a whole and specific sub-groups of students (racial and ethnic minorities, English Language Learners, and disabled students, for example). Nothing about the growth model encourages public schools to make sure that students who are already meeting or exceeding proficiency have to stretch.
Some of us renegade teachers do it anyway. If we are not teaching Reading, Language Arts, or Math, we can usually get away with it.
It will be interesting to see how the Common Core standards affect schools, since they are geared a little bit more towards critical thinking than most of the older state standards. As a social studies teacher, I find that I have to change very little about what I teach to accommodate the Common Core standards. What I used to call a stance, I call a claim. That's about it.