Retention for two years is generally too much.
You don’t want an 18 year old with 2 more years of high school.
Retention is a very nuanced conversation where the camps tend to talk too dogmatically.
There are the “all retention is bad” groups, and there are the “all social promotion is bad” groups.
This is exacerbated by the fact that some states have a “pass the test or get retained” policy.
The research matches my professional experience, which is as follows.
Retention as an intervention is not effective.
If a child’s learning curve is not keeping pace with classmates, retention will not help.
The child needs interventions that will get them on the road of improvement first.
Then,if you have an intervention where a child is learning and trying to catch up, but they are so behind that their rate of improvement cannot over come the headstart that the rest of the class has on them, retention can be considered to help overcome close that gap. If a child is 2 years behind class average, even if they improve at the same rate as everyone else, they will always be two years behind. (This gap tends to close in high school, because 8-10 grade reading level is more than adequate to access high school curriculum and the real world. Not being able to read Shakespeare isn’t the end of the world.)
Other things to consider… is it just academics, or is the child physically/socially less mature?
People love to retain in Kinder, but it just gives them two years of Kinder, where if you wait to retain in first grade, they get two years of phonics instruction.
It works best in the “learning to read” grades (1-2, sometimes 3rd.)
It works least in the “reading to learn” grades 5+ because repeating the classwork may not necessarily address the reading skills instruction.