system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it negative to hold students back in school?

Children develop at different rates, so what does it matter if a student wasn't ready for, say, fourth grade? Why is this stigmatized? 

This may snowball into a 'philosiphy of education' thread, but that's okay. Education is arguably one of the most important foundations if society, and it's an worthwhile discussion. 

Why do we in America push marginal students through school?
gpt
Basically, school, particularly primary/elementary school isn't just concerned with academics. A good school should be trying to nurture fully rounded human beings and a large part of that is found in the social sphere. Separating a child from his peers can cause anxiety, depression, a drop in self esteem etc etc. generally it's viewed that boost in academic learning is not worth the trade off in the other less tangible areas. Particularly when most studies show that the child who repeats a year generally find themselves at the bottom of the class again within two to three years.