I sympathize if she lost friends. That being said, no Christian school preaches hope for death for anyone. They teach you to pray for your enemies.
If this person is telling little children to hope for the killing of someone, the person is letting their personal feelings overcome their common sense. It is clearly inappropriate in my view and I would make it known to the teacher and inform my child that the teacher is incorrect.
None of this has anything whatsoever to do with the murderous Bin Laden.
I never took it that way, that is, we should personally want to kill someone. She says "I hope THEY Kill OBL". What you said did first cross my mind, but I've always taught my kids the Gov't does not carry the sword in vein. They encourage those that do good, and punish those that do evil. (Romans 13) (ie, Gov'ts are ordained by God to carry out Law and Order)