The way that the Japanese schools handle it to my knowledge is they never have kids falling behind grade level unless they are in the hospital or out sick for a long period. They usually have a four or five student team (gumi) with students of varying skill levels, so the whole team works through the assignments, helping each other along the way. This operates much like the old one-room school houses did long ago in the US. The “older” students teach the “younger” students what they have learned previously or during that lesson. The whole team moves together. This way everyone stays at grade level and passes to the next grade.