Technically, no. The value isn't for the children it's for the educators and the assessors so that they can see if person X knows what they are expected to know by the end of Y time. Because education is not personalized for the most part the blocks of time used are arbitrary but grant enough time for "most" to get it though that in and of itself is likely not actually scientifically verified.
So you're right, in a sense, but it has to be that way so that we can tell who is and isn't learning what through scheduled assessments of some sort.
There is no reason to have a "grade" in school because it's basically based on the teachers opinion. The teacher could have favorites and let them have an easy ride. Not all teachers are good, but the ones that are good try and help the ones that need help. The students that move around will have a different knowledge of something. One who lives in California could be behind on a subject than ones who live in Florida. The curriculum of education is different. Why are teachers teaching different things? Yes, teachers have different ways of teaching, but putting their opinion in something shouldn't happen. Teaching exactly to the book isn't productive. Getting the students involved and wanting to go to school is being productive. Taking field trips is a great way of learning about a subject. Doing hands on activities will strengthen the knowledge of the subject.
There is no need to stretch school years for so long. Particularly in the likes of high schools. We could very well have teenagers who are ready for work at a younger age than now. This will no doubt have a greater effect on economies and we can still come up with other ways to protect the children.
They could easily have it set up to get through multiple grades a year. Its in the younger years to Middle school where they slow down so much that children start losing interest in learning. Children naturally love to learn but if you repeat knowledge they mastered over and over again; the children become complacent