Showing Horse Crazy by Bonnie Bryant

Twelve-year-olds Carole Hanson and Stevie Lake have been best friends ever since they met at Pine Hollow stables. So when thirteen-year-old Lisa Atwood shows up for her first lesson dressed in fancy riding gear - and acting chummy with the snobbiest girl in town - the girls aren't sure she'll fit in. But Lisa soon shows herself to be a quick learner, and Carole and Stevie can't help but admire her natural riding talents. Soon the three girls are fast friends. They begin to make plans for the MTO, the long-awaited Mountain Trail Overnight campout for the students of Pine Hollow. There's just one problem: Stevie's parents won't pay for it unless she improves her grades...