Our DS' school gave him the end-of-year tests for 3rd grade (he was in second grade at the time) and then 4th grade math. When he passed the 3rd grade test (80 some percent), but not the 4th grade (50/60 percent), they moved him into the 4th grade class for math only. It worked out quite well. It took some schedule adjustment but both teachers were more than willing to make it work (and so, it did. lol). Would that approach work? It isn't all that uncommon where we are, I know or have heard of other children who are accelerated for one or two subjects.