They may avoid one another because they are afraid of saying the wrong thing. Have a group meeting to discuss appropriate and inappropriate things to say in times of grief, and what to write on a card. A good setting to talk about grief and comfort is in making cards together. Invite them to work in pairs or small groups to find scripture they think is comforting. Invite them to share verses they've chosen and talk about why they find them comforting.
Teens speak candidly and with faith about their own losses.
It is quite powerful and moving, and might not be appropriate at the time of a community crisis. As with all resources, preview before showing to a group. An excellent pamphlet with guidelines for using this video for discussion accompanies it. When Someone They Love Dies. Theresa Huntley, Augsburg Fortress, 1991.