I think I may have imagined a kid I went to school with and now I think I'm crazy. Has anyone ever had to question their own sanity?

sorry if this story is a little long.

I was in the 6th grade and about halfway through the school year a kid came to join our classroom for the rest of the school year. His name was Kevin and I think he had down syndrome or some sort of mental disability. I'm not sure why they didn't have him in special ed. I think the story was that they thought he would develop more socially if he were with "normal" kids

Kevin was certainly a character. he would make random noises, I think he said they were train noises, and to be fair he did sound like a train. EVERYONE loved him since he was really nice and liked to talk to everyone, he was everyone's friend. He took to one girl in particular and she took to him. They would always hang out in the class room, at lunch, and at recess. What made it even better is that she was one of the popular girls and she gave zero fucks about what people thought of her and Kevin.

Near the end of the school year he suddenly disappeared, I think his parents moved or put him in a different school. Everyone was sort of bummed out but the girl that he always hung out with was CRUSHED. I remember her going to the back room and crying for like an hour while her friends comforted her. I know this may sound weird but I think Kevin helped me better understand the mentally handicapped and made me think of them as people too.

Fast forward to high school. I'm hanging out with some of the people who we're in that class with me and for some reason Kevin pops in to my head. I say something like "Hey remember Kevin? Does anyone know what happened to him?" and they all look at me like they have no idea what I'm talking about. I try to describe him and they just look at me like I'm crazy. How did they forget him? he constantly made train noises, how is it possible to not remember that?

So, I go to up to the girl who was his best friend and she also has no fucking idea who I'm talking about. I must have went up to almost every person who was in that class with me and NONE of them remember Kevin.

And here's the weirdest part. Since he came to our school mid year he wasn't in the class photo and since he left before the end of the year he wasn't in our schools year book. So there's zero evidence of his existence.

Now I think this kid never even existed and I'm insane.

TL;DR Went to elementary school with a very noticeable kid and no one remembers him except me. Now I think he may have only been a figment of my imagination

Edit: Spelling