The Edge of Forever - Margaret Pearce - Bog

Jane Smith is the unhappy, resentful, eleven-year-old daughter of a single parent. Her escape from the unpleasantness she considers her life is through another world. There, she's one of the four powerful Patrollers sworn to the service of the Princess in the Far Tower. Their quest: To choose the bravest warriors in all space and time to fight the Red Wizard.
Jane's ordinary life improves as she makes friends. She's picked for the school play and the prestigious trampoline team. But her otherworldly life with its epic adventures continues to be more important to her than the one she lives in...until the warriors' battle causes a merging of Jane's ordinary world and her otherworldly one.