I would call her parents to pick her up. its too dangerous to keep her in care. She could get hurt or the other kids could start doing it too.
I would also put her in a playpen close to were i was, so if she does faint she is in a safe spot and I can keep an I on her.

has she already fainted, or are you just afraid she will? One can "hold their breath", but really still breath throw their noise. I have seen a kid do this. It looked like he was holding his breath for an hour, but he really wasn't.