Fold 12 sheets of foil, each 12 by 20 in., in half crosswise; then oil tops. Center 1 graham cracker on each oiled doubled sheet, then top with a chocolate piece, marshmallow, and another cracker. Gently fold foil over s'mores and crimp to seal.
Heat packets on a cooking grate over glowing coals in a campfire or on a grill over medium heat (about 350°), turning often just until chocolate softens, 2 to 3 minutes. Or, using tongs, grasp packets on sides and heat over a low fire.
Make ahead: Through step 1, up to 3 days, airtight and chilled.
Note: Nutritional analysis is per s'more.