Social promotion is the policy that you are moved along to the next grade in public schools even if you fail your classes. There are two main justifications for this policy:

1. The schools do not have the resources that would be required to actually teach the abundance of kids that would be held back in the grades if they didn't allow them to progress to the next grade.
2. It's not fair to shame the kids who fail their classes by holding them back a year.

Here's an interesting piece of history on the subject that I didn't know about until now. The bold is especially interesting:

With the proliferation of graded schools in the middle of the 19th century, retention became a common practice, as were mid-term promotions. In fact, a century ago, approximately half of all American students were retained at least once before the age of 13.

Social promotion began to spread in the 1930s along with concerns about the psychosocial effects of retention. This trend reversed in the 1980s, as concern about slipping academic standards rose.
Personally, I'm of the belief that people in power want to weaken those who aren't in power to be able to control them and profit from them. Social promotion (along with a number of education policies in the United States) seems to do a great job of that.

Thoughts on social promotion specifically or the education system in the United States being designed to make people easier to control?