This eighth-grader stayed back once or twice, to be 15 and in Grade 8.
Standard age for 8th grade is 13 when the school year begins, usually 14, except those with July/Aug birthdays, by the end of the year. That makes a 15-year-old one year older than “base,” although there are more kids that age, especially boys, these days, because parents are using “transitional kindergarten” and other ways of having boys start first grade when they’re older. (This provides academic and especially sports advantage for the boys.)
(/not particularly relevant school-type info)
It’s possible he came from Mexico (ie was not born here nor was he a citizen) fairly recently, with his parents, and thus was behind. I had students when I taught in TX (8+ years ago now) who had come over as older children and were woefully behind, plus didn’t speak English. Just a thought.