The government, in a show of goodwill, has offered a very extensive training grounds to the Justice League.
Granted, the offer is to utilize them, not own them, but they are amazing.
A virtual reality so real, run by an AI that truly generated random conflicts, that it's absolutely perfect to train child heroes with no actual risk to them.
The world is always a bright, vibrant green, with floating purple doors.
The trainees are given specialized suits and weapons, so that they can fight back and feel the hits. They are loaded up into a "transportation unit", to give them the feeling that they're actually going to a location, not just waiting for the AI to boot up the VR room.
After that, they fight whatever the AI spits out.
If the fight is too much, all they have to do is turn off the suit, and the environment goes right through them instead of hurting them.
The weapons alter between "containment" and "destruction". Some of the "missions" given by the personnel in charge of the training room, the GIW, are for destruction of all foes. Other times it's for capture.
They fight strange beings with really odd quirks; a rock star, a giant metal man, an evil therapist, est.
However, when they get too close to capturing or destroying the target, there's a 60% chance that the "boss" appears.
The boss is a challenge, because the only thing trainees are allowed to do to him is capture, but he's insanely powerful.
An AI bogey called "Phantom".
Phantom has never, ever lost.
But some trainees are getting suspicious.
The ones that have been dead before, when they turn off their suits they can still get hurt.
Or; I was inspired by Enders Game.
The GIW make their own portal, and start advertising it as a "training room" for heroes. The plan is to make the heroes decimate the Ghost Zone, but they didn't account for the adult heroes to take one look at it and think 'oh, that's perfect for my mentee' instead of use it themselves.
As they're not their fully trained counterparts, this means that when Phantom enters the picture, instead of him getting beat by the professionals as the GIW intended, he dukes it out with kids his own portrayed age.
Naturally, in his home environment, he wins.
So the kid heroes, dealers choice, slowly come to the horrifying realization that they're being used as weapons to attack another dimension.
