@SuzyQ7, if a student repeats once, he or she may be 16 or 17 as a sophomore, not 18. Sometimes, the “repeats” were skipped ahead a grade or two in earlier years, so they’re 14 or 15 as freshmen. It’s usually a family decision, rather than a school decision, but if 75% of the boys are repeating 9th grade, the school seems to like older boys.
For individual boys, repeating can make sense. Boys start puberty at vastly different ages; I can think of some kids I’ve known who were not ready to board as 8th graders. If a child has had a serious illness, such as mono, it can make sense to repeat a year. If a school gets a reputation for its population skewing older, that can reinforce the pattern, of course.
I don’t think it is a good policy to repeat a grade just to gain a leg up on peers. For one, I don’t think it works. The academic stars are often “young for their grade.”