There was a boy who was in the pictures of my first grade-mates (that is, the kids in my 3rd grade class in public school, back when they were in 1st grade), and then in first grade again the next year. He had dyslexia, but it didn’t get diagnosed like it does now, and so he got held back for not knowing how to read-- at all. His parents put him in a summer program for dyslexics at their own expense, and he did much better his second year, but he was always the star athlete in the class. He wasn’t a lot taller than the other boys-- he might not even have been the tallest-- because he was pretty young, with a summer birthday (they lived just two houses down the street from us), but he was by far more coordinated, and strong, and was on all three teams in middle school, plus picking up wrestling in high school, then getting a wrestling scholarship to college. I’m not sure how his age affected his eligibility in college. It may not have, since he had a summer birthday.
Anyway, when he was first told he had to stay back, it was probably demoralizing, but all in all, he did very well.
I also knew of a kid in my jr. high who got retained who was a foster child, and got transferred, and missed a lot of school, because he parents were trying to get her back, and she had to go to hearings. I don;t think she ended up graduating, though.
There was one girl who secretly impressed me, though. She got pregnant, and missed a year after she had her baby, but then she went back, and finished. So she finished at 19, but she finished. She even said at one point that she could be a better mother with a high school diploma.
I’m not sure what the law is. If you are disabled and on an IEP, you can legally go to public school until you are either 22, or have a high school diploma. I don’t know how old you can be and still attend if you have not attained a diploma, but are not on an IEP (that is, disabled under law). But you definitely can attend until 19 in the state of Indiana, especially since the revamping of the kindergarten curriculum, causing lots of students not to be able to start K until age 6 (some systems screen every single kid). I’m presuming you can probably attend until you are 21 even if not on an IEP. But I know that PL 94-142 (The Education of All Handicapped Children), which is a federal law, says 22 for those on an IEP. And they don’t get kicked out at 22. They can attend until the end of the year in which they turn 22. My son has an October birthday, so were he on an IEP, he could attend until 22 & 7 months, since the year runs from Aug. 5 - May 22.
Kids who are retained a year are not necessarily on an IEP, but kids who are retained more than one year probably are.