I know both situations. Being kept hungry for knowledge and understanding and underfed due to being faster than the rest and being slower than the rest due to gaps of knowledge that cannot me closed on-the-fly while processing new and overwhelming things. From that experience I would say that children with a fast pace need a different treatment compared to those with more average pace and the slower ones too need other types of approaches. As soon as you separate those groups you also create social groups who will start with vanity, envy, hate, condescension. No good. Better to keep all parties united und give those who need it some extra care and support. When you have disabled kids it is important to keep them among the group. So the group has a chance to accept them as valid members and support them too. We do not want KZs of any kind, do we?