As part of his series on New York five years after the 11 September attacks, the BBC's Stephen Evans examines the psychological impact on a particularly vulnerable group.
The mental scars of children in New York are often just below the surface - but deep nonetheless.
There was a cluster of schools around the site of the World Trade Center and many of the pupils were shepherded across the area during and after the attacks.
Take the case of Obinna Onwuchekwa, who was 16 at the time and a pupil in a school literally in the shadow of the towers.
He is now cheerful when you talk to him, with the front of a young man setting out in the world.
But hurt hides behind a brave face: "When I was evacuated from the school, I looked up and what I saw was a woman waving a little towel and another person who literally jumped out the window.
"And I was thinking 'this literally isn't happening. Maybe this is a movie'. But it was for real. People were jumping.
"And that's one of the memories I always had nightmares about."
He told the BBC the effects are still there: "I'd be lying if I said I'd got over 9/11. It affects me every day. I'm one of the few people who witnessed 9/11 up close - it's horrifying for me.
"I have those memories for the rest of my life.
Obinna says he still cries a lot when he tries to talk about the attacks.
"Sometimes I'm very depressed - I kinda thought about suicide. My grades went down. I was a very smart student but you wouldn't see that if you looked at my grades after the tragedy."
It's not uncommon in children in New York.
Dr Robin Goodman is a psychologist who has worked closely with children who were bereaved or who witnessed the terrible events.
"I worked with kids that had nightmares or were afraid that something bad might happen to their parents," she said.
"They saw things because they were in school and the windows looked out at the buildings.
"Some kids were afraid to go in the subways or underground because they were scared that something would happen".
She has published a study of art created by children in New York. The images are very graphic, with the towers burning and people falling from them or planes crashing into them.
One picture shows a caricature of Osama bin Laden snatching up the towers and stuffing them into his mouth.
The difficulty for some children was that they started to associate all the experience of the day - like meeting a teacher or just going to school - with the attacks, so Dr Goodman's task was to help her patients dissociate the innocuous events from the hurtful ones.
"One of the things you do is help kids learn how to manage those upset feelings, to develop skills to feel more relaxed, to feel calmer, to feel more in control.
Robin Goodman: Coaxing children back into school
"What you do is gradually expose them to what happened, and as they're able to feel more in control, they're able to then reclaim their life and handle the trauma and the reminders that much better.
"So you help them to, first, take out a math book and then actually go by the school, and then actually go in the lobby of the school, and then actually sit in the classroom, and then actually sit in the classroom with other kids".
"As you help them, they're able to go back into those situations and then re-engage in their life."