#4 Mandie and the Forbidden Attic by Lois Gladys Leppard, 1985.
Mandie is unhappy because her mother and Uncle John are sending her to a boarding school in Asheville. It is the same school her mother attended years ago, and her grandmother lives in the same town. Mandie is upset about leaving all of her friends and family behind, but Uncle Ned says that he will still visit her.
By the time that Mandie arrives at her new school, everyone else already knows each other, and none of them seem interested in making friends with her. One girl in particular, April Snow, takes a dislike to Mandie, which is difficult because they have been assigned to the same room.
Things change when Celia Hamilton comes to the school. She and Mandie become best friends, and Mandie is allowed to share a room with her. Then, strange things start happening. Mandie and Celia keep hearing strange noises during the night from the attic floor above them, and some of Mandie’s clothes disappear. At first, they suspect that April Snow may be playing a mean trick on them, but it turns out that she is not actually at fault. There is someone at the school who doesn’t belong, but finding this person may get Mandie and Celia into trouble.
This adventure requires Mandie to meet with her grandmother again. Mandie is nervous about seeing her grandmother because her grandmother helped break up her parents’ marriage and sent her away with her father when she was a baby. Surprisingly, Mandie’s grandmother seems to like her and her friend Celia. She admires their spirit and even says that Mandie reminds her a little of herself when she was young. This meeting helps the granddaughter and grandmother to make peace with each other and the past.