Actually when I was in school many moons ago, we had a 17yo in 9th grade. I knew Fred, he was a nice guy just a tad retarded. He got drafted and got seriously hurt in basic unfortunately falling off the cargo net on the obstacle course.
There was no social promotion then.
We had a few slow students at my small town school, but they were usually withdrawn and sent to a special ed school. We had a kid in 3rd grade who was twice as big as we were. He was probably 13 or so when he came in as a ‘new’ student when his family moved to town. We could tell he was ‘not all there’ even though we were only 9 or so. He lasted about a month before he was sent elsewhere....................