D.B.
Kids aren't qualified to diagnosis Dissociative Identity Disorder (multiple personalities) and neither are adults. What's going on here is bullying and most schools have an anti-bullying policy. Let the principal know what you heard, then let it go.
Mood swings are a reality for many people but talking about someone as if one is an authority on mental illness in general or this child's mental illness in particular (if there is such a thing) is just gossip among the kids, and it needs to stop.
And by the way, the book "Sybil" has been debunked once the therapist admitted that she made it up.