This is actually my daughter's idea, but she's too young to post. She's passed through elem. in this district, and thinks she has a solution that would greatly improve K-6.
She thinks all grade levels should basically go away and classes should be structured on competency or ability. So high level reading kids take reading together and low level math kids are together, etc. I know a lot of parents would revolt, but she has good reasons for it.
One, teachers can not be there for every level of kid in the current system. Even the best teachers, some group always gets left out. The adv. kids sit there and do nothing or work on more worksheets, never really getting the extension and stimulation they need, while the struggling students might get help, it's usually fragmented and they continue to struggle.
Two, kids are very self-conscious in mixed groups. The adv. kids may know the answers, but don't get called on because "everyone needs a chance", which sends a negative imagine to the adv. kids on their worth. Again, they're being held back. The struggling kids don't want to call attention to questions they have, so they often sit quietly, even though they may be lost and confused. Or, they act out in other ways to bring humor or distract from the lessons. Both situations are bad.
When they're with their academic peers, all can shine and work together to stretch and succeed. Teachers can focus their efforts where it's needed. If grade levels were eliminated, you could accumulate points toward "graduating" elem., kind of like HS, I guess. But you don't "have" to be in "5th" grade just because of your age. You go where you need to be to learn the best. That may mean you have classes with a wide variety of ages.