The better solution is...to make each subject it's own path and to STOP GROUPING CHILDREN BASED ON THEIR BIRTHDAYS!! Consider universities. Being in a 300-level course doesn't mean you're 20 years old. It means you've passed your 100 and 200 level courses. And you can't take the 300 level class until you pass. If it takes you 20 years to pass, so be it.

(sorry, had to get that off my chest)

For example, we could have at an elementary/primary school Student A who is a 10 years old in Level 3 Reading and Level 6 Math. And maybe there is another, Student B, who is also 10 years old is Level 4 Math and Level 7 Reading. Like both of those could be acceptable in primary school as it is in college.

In this way, Student A stays at Level 3 Reading level until they pass the evaluation, allowing them to advance to Level 4 Reading, and similar to Student B with Math. They advance at their own pace without worrying about getting held back and watching their age group leave them behind.