going on a limb to say niehara's true form would be something related to the earlier chameleons. itou points the detail that there werent that many in the first place, so the teacher could have been replaced by one while the rest sit waiting to replace more of the class in the tanks. would also explain why he say itou killed a 'subordinate' at the end, rather than killing a piece of himself, which was my original line of thought. also, the apparent parallel between their camouflage and the 'type' of monster he classified himself as.
I'm thinking something similar. When Niehara said their apparent body is "like a movie screen", they also said their real form was Slenderman sized. Except Takahashi would've noticed if she was biting into something larger, and I don't think Slenderman could've fit into a crowded field trip bus anyway. There's also little moments where Niehara gets a weird amount of things done at a time, or seemingly reappears from nothing. I'm pretty sure the Niehara we see is a puppet body, and their actual self is an invisible, chameleon-y stalker nearby. Which makes some of the imagery pretty on the nose!
This is also why they said Itou can kill them: once you figure out the trick, you could do them in with nothing more than a fire extinguisher. Spray around to find the invisible thing, spray some more to debilitate it (probably incapacitating the puppet in the process), then get very insistent with the canister and Slendy's cranium. This is probably why it took Niehara so long to do in their abuser, too. Without an appropriate frame of reference for what humans are capable of, all they could see was their own frailty.
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