@Rothbardian_fanatic has answered the Japanese context adequately, so let me fill you in on my country’s situation xP

Oh we have zero qualms about holding students back if we think that they are not up to par. Let me scope our realities in the example of pre-tertiary education. We have two years of high school education, termed junior college one and junior college two. When our 18-year-old teenagers are in JC2, they need to sit for the GCE Cambridge A Level exams. So, many junior colleges retain their JC1 students (can be as many as 100 odd out of a cohort of 700-800 students) for one year because the teachers deem that they are ill-prepared for the rigours of this pivotal life-changing exam.

I think the Asian concept of “saving face” has a role to play here. Being a small country, we all know which junior colleges perform the best. So zealous principals and middle managers mayn’t want to send their weakest students to the battlefield because they will lower the college’s median grades haha. We are a rather competitive society here

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.