It would be easier, more academically sound, and even more economical to prevent dropouts by retaining these children in the primary grades. Imagine going to your physician with an illness. Everyone will have negative side effects, some of which can be severe, some lifelong. The illness is underachievement. It's easy to understand the appeal of holding back a student who fails to meet benchmarks.
Retention can take a student from the bottom of a class to somewhere nearer the middle.
The problem is that students are compared to the grade placement, not to their peers. Most likely, the learning problems that contributed to the original retention decision will persist, and the retained students, in a year or two, will be back at the bottom of the class. As a district wide school psychologist, I follow retained students for a few years. In my experience, grade retention is a dangerous gamble. But you don't have to take my word for it.