Why Aren't Schools Grouping Kids By Intelligence Level?

As we look at ways to improve and update educational systems, does it still make sense to have kids grouped by age rather than by their level of intelligence? Should less intelligent kids have to be overshadowed by geniuses? Should Geniuses have to wait for dullards who may never catch up? I can't remember all the times I would see the really smart kids in my class play card games while the rest of the class was learning stuff these kids had already mastered. In professional life the better workers get to advance past go, why shouldn't the really smart kids be in the same class that goes as fast as their minds can?