Between the books in 11-year-old Dylan Wilson’s backpack lies the accessory other students at Orchard Grove Elementary School most covet: a plastic necklace with a kaleidoscope of pendants in the shape of shoes. He has 66 pendants so far, one of the largest collections at a Frederick school that has developed an uncommon yet effective fascination with running. The school’s “mileage club” started in fall 2009 with the idea of handing out the charms as an incentive for kids to run during recess.