Sue the living hell out of them.
Particularly if it is a public school. I would contact an attorney, then the local news media, and make an example of them.
However, if your young kid was spending the night on a field trip somewhere, I doubt that it was a public school.
If it were me, things would have been all OK if they had notified you as soon as it happened, taken the kid to a Dr. if necessary, and removed the dog. But, they ignored it, or worse - tried to cover it up - so I would be very aggressive with them now.
