Found some interesting commentary and link on a fellow blogger's site, regarding teaching to the development of the learner rather than grade placement based on age.
The End of Grade Levels
I believe there are solid pluses to this idea, which are some of the same reasons multi-age classes can be powerfully successful: less stigmatism for meeting, or not, some benchmark that has been placed on that grade level; the opportunity for students to learn more from each other; and teach kids to learn and truly work with others rather than segregate based on age. A discussion I started to have frequently this spring with some of our students was that only during your school years will you be surrounded by people your own age. In the workplace, or even college, you will have folks of all ages in your classes or environment and it is better to learn how to deal with both the positive and negatives of that, than it would be for me to not schedule freshmen in the juniors' classes when needed.
And, as with anything, there are certainly some negatives to prepare for: the stigma of not "keeping up" with peers will remain depending on how actual groupings are created; the the application of the concept so that slower learners are challenged and don't end up floating through an even bigger black hole of education.
Done correctly, we can instill a solid love of learning and build self-esteem which only benefits everyone. Many private LD schools have achieved this for quite some time. The challenge is how to set the expectations, implement them AND have people feel comfortable trusting that system.
Change is challenge and excitement all rolled together!!!