[directevolve]

If a kid achieves a great 8th grade test score at age 18, is that a success or a failure of the system?

What we care about is the level of achievement by a given age. To determine that, we need to be comparing states using standardized tests given to age groups, not grade levels. It is fine to hold students back, if we think that will do them more good than advancing them. But they still need to be tested the same way as their age group if we want to do a meaningful comparison between states.

[ryathal]

If an 18 year old achieves a great score on an 8th grade test they are above average for adults.

[seanmcdirmid]

If the kid is held back and not failed forward, at least they get a chance to fix things.