Retention is a very loaded topic with a lot of emotion.
Some things to consider.
1. The research base is very negative on retention, but the data set is generally huge population sets of people who do it right and wrong. Generally speaking, the data is skewed by states and districts that blindly retain off of a test where students haven’t met a cut-off score – which is exactly the wrong way to do it.
2. Retention is not really an intervention unless done in 1st grade when a student receives two years of direct instruction of basic phonics and math skill development. This is challenge in grades 3-5 where the students are transitioning from Learning to Read to Reading to Learn. An extra year will not equal more direct instruction in those missing skills because it is unlikely that they are being explicitly taught anymore. And of course, distance learning and COVID has complicated anything related to the last year.
3. When I have seen retention done correctly in grades other than 1st, the student is receiving excellent interventions and is improving, but they cannot close the head start that their classmates have on them to catch up. If the student is not receiving those interventions and improving at the same rate as their classmates, retention only delays addressing the problem another year – which does more harm than good.