The events of the last two months are overwhelming to adults; just think what they must be like to a six-year-old. Monitor your daughter's TV viewing and remember that there is nothing wrong with turning off the news when she is watching. Children are not able to put events into perspective like adults are, and video clips of war or of buildings collapsing may seem to be just down the street instead of hundreds or thousands of miles away.
Be patient with your daughter and continue to tell and show her that she is loved. Reassure her that your family is safe and that you will always love her and each other. Work out a plan with your daughter's teacher to address her behavior problems at school with firmness and consistency; she still needs to know that the rules you've had in place are still the rules she must follow.