Charlotte sometimes

It is Charlotte's first night at boarding school, and as she's settling down to sleep, she sees the corner of the new building from her window. But when she wakes up, instead of the building there is a huge, dark cedar tree, and the girl in the next bed is not the girl who slept there last night. Somehow, Charlotte has slipped back forty years to 1918 and has swapped places with a girl called Clare. Charlotte and Clare swap places ever night until one day Charlotte becomes trapped in 1918 and must find a way to return to her own time before the end of term.