They take him in for an evaluation when he's five. He doesn't quite understand what they're doing, but he goes along with it because if he passes it means he gets into the special class of ninja who will be training in secret, and that means it's the best one.
When he finishes the test--it's boring, mostly talking--he listens in on the conversation the two jounin are having.
"Some potential sociopathic tendencies, it seems." (Deidara doesn't know what this means.)
"When he gets older, you mean."
"Mmhmm. And he has potential." (Oh, maybe that was a good thing, then.)
So he got in. The class is small, perhaps twenty children, and he doesn't know them all but he's at least heard about them. Tomoko-chan lives next door and had her throat cut out by a Konoha-nin, so she can't speak, but she's got dark, watchful eyes and more skill with kunai than a lot of genin. Yuuto-kun he's never seen before, but everyone knows him, with that stiff tone and the rumors he'd killed three children who had been bothering him. Akemi-chan is laughing loudly with another group of girls, but everyone knows she was born on the battlefield, ripped from her mother's womb as she died and hidden away until they could bring her back to Iwa. No one knows if it was the premature birth or her surroundings that made her so vicious, but she was easy to get along with usually.
Deidara made friends with those three, because he thought they were the strongest aside from him.
"Deidara-kun, don't you have a sister? Bring her to class~!" (She's joking so he doesn't say he'd kill her if she touched his sister.)
"You just wish to see Tomoko-chan and Deidara-kun get into a fight. We are not supposed to kill each other."
"I don't make Tomo-chan use little kids for target practice." (Deidara doesn't hold it against Tomoko-chan because she knows better than to throw anything at Miu.)
They go for evaluations once a week, and are advised on what to do to improve. Deidara thinks these are boring because they don't look at how fast he's learning the skills but how his mind is developing, and talking to them is boring.
They just tell him how to properly deal with other people, not that he needs encouragement. They tell him they're strings to be cut, tools to be used and thrown away, and he's always known this on some level, they just bring it out more. They tell him to focus on himself, they tell him that he doesn't need a conscience when Iwa will tell him what to do.
"So you killed your mother?" (Dammit, they knew about that? Oh, he was dead, no one would believe it was an accident even though it was one.)
"You're progressing nicely. She didn't matter, right?"
"Quite ahead of the class, aren't we, Deidara? Good job." (...Well, he wasn't in trouble. Whatever.)
He keeps receiving the training even after he's graduated into the Hunter academy--mostly the evaluations, although they've slowed to once a month now. They mostly just make little "hmm"s and nod and tell him he's developing well, and when he becomes a Hunter the evaluations stop entirely.
He's still with Yuuto, Tomoko, and Akemi, because they work in a unit for their hunts.
"Akemi what are you doing? Are those intestines?" (They've killed three targets, and the cleanup is going slowly, so Akemi's bored and throwing chunks of one of the bodies over to Tomoko for target practice.)
"Maybe~."
"Organs are not decoration. Do you know the diseases you could ge--Deidara, stop encouraging her!" (She's also wearing the intestines as a necklace, which seems hilarious to Deidara.)
This is the training of Iwagakure.
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[The Hitomi records a moment of Deidara laying unconscious, and then switches off.]