When I was in the lower 6th form (in the UK, age 16-17), we had a scheme where L6 kids would be paired up with kids in year 7 (age 11-12) to do one-on-one reading with them, discuss the books with them, help them with any words they didn't know etc. The kid I was paired with was pretty smart and didn't really need my help, so I got a quiet 40 minutes once a week. But one of my friends was paired up with a kid called Tom.
Something was really, really off about Tom. He looked Mongoloid, but because he had an Asian surname he may have got his epicanthic folds the normal way. I'm pretty sure he had some level of autism, but he didn't get any special ed provisions as far as I knew (and in the UK you'll get special ed provision out the wazoo if your kid is in any way subnormal, it's not hard to get if you need it), and it looks like his parents decided to mainstream school him either out of denial or a misplaced desire to socialise him.
Whilst my kid burned through three Willard Price books over the course of the school year (Who else remembers Willard Price books? They were equal parts ludicrous and badass.), Tom didn't manage to get even half way through his first book. It wasn't through lack of reading ability, just a complete inability to concentrate on the book, or even on reality. Trying to talk to the kid would result in a non-sequitur. This kid just parroted phrases he'd heard like a Mynah Bird, completely out of context. He had a number of phrases he'd use constantly. I remember one, "Go on Shag, have a go, he's off his line!", because it was from a Nick Hancock football bloopers video that I happened to own. But his favourite was "Always leave the water running", said in the same tone of voice as "No wire hangers" from Mommy Dearest. I have no idea where that was from, but if you asked him a question there was about a 25% chance that this would be his reply.
He was usually very calm and quiet, but would have random outbursts of emotion that were completely out of control, over trivial shit that was never possibly to predict. One class the teacher asked the kids to design the front cover for some short booklet they were supposed to write. 5 minutes in, Tom takes his "artwork" to the teacher. The teacher asks what he wants, and he holds the paper up to her, says "I don't like it!" and burst into tears. This then became a screaming tantrum necessitating him being removed from the building. The even weirder thing was that he hadn't drawn anything yet, just a square border.