n. The practice of holding a child out of kindergarten for one year in the hope that the child will then do better academically and socially.
Other Forms
About 9 percent of 5-year-olds nationwide are 'academically redshirted,' or held out of kindergarten for a year, according to the National Center for Education Statistics. …

Academic redshirting is a result of higher expectations of kindergartners, some educators say, and makes sense for students with summer or early fall birthdays, or for ones, usually boys, too immature to handle kindergarten.
—Katherine Cromer Brock, “Redshirted but ready,” The Wilkes-Barre Times Leader, August 10, 2006
The rise of redshirting dates to the 1980s, when mandatory testing put pressure on schools to make the early elementary grades more rigorous. As 3rd and 4th grade became more demanding, kindergarten had to keep up. Savvy parents were increasingly likely to worry about their children's early academic performance and resort to redshirting, Meisels says.

A parallel national trend of moving back the cutoff age for kindergarten eligibility, making the average child older when he or she began, also contributed to the "graying of kindergarten."
—Nara Schoenberg, “Postponing kindergarten,” Chicago Tribune, April 26, 2006
1985 (earliest)
Parents who "redshirt" their 5-year-olds instead of enrolling them in kindergarten are a concern to some Nebraska educators who are trying to reverse the trend of holding children back until age 6 to start school.
—“Some Educators Oppose 'Redshirting' 5-Year-Olds,” The Omaha World-Herald, March 01, 1985
This term comes from the common practice of "redshirting" freshman college athletes, which means keeping them out of competition for a year to develop their skills. This sense of the term dates to about 1950.