Cheerleading was at the center of Laura Jackson’s life since she began shaking pom-poms for a pee-wee football team in the third grade. At 14, she dreamed of cheering in high school and then, maybe college. But on the day of tryouts for the freshman high school squad in Livonia, Mich., those plans were shattered. That afternoon, as her turn arrived, she got ready to perform a back tuck, a challenging gymnastics move she’d learned just for tryouts. She eyed her spotter, a girl just three years older than herself, and took a running start across the gymnasium floor before...