Twice Upon a Time by John Pinkney

Sara and her sister Ella have been  full time boarders at St Ann's College ever since their parents died in a car crash and their grandmother had become too frail to adequately care for them. Ella is a sad little girl who is finding school difficult and who is about to be moved to the new junior campus. Sara has been sleepwalking and is also very concerned for the welfare of her younger sibling, and is desperately trying to shield her from the unorthodox disciplinary measures of the junior headmistress, Miss Sharp. One day Sara discovers a strange wooden door  beneath the school at the end of a maze of tunnels. This door offers Sara some optimism for the future, but that which is on the other side is not always the happy life Sara discovered the first time she passed through it. The book looks at the possibility of parallel universes and is a hard book to put down. 

The House Guest by Eleanor Nilsson

This book has been rated as a top read for boys and girls would love it too. It's another Australian gem I couldn't put down. Gunno and his gang regularly skip school and raid houses for fun, stealing cash only. When Gunno and the gang break into The Big House, he finds that his life changes as this house in the valley is different and it holds some fascination for him. He is continually drawn back to it. and starts visiting regularly thinking of it as his house; he loves the little dog there and day by day is learning about the lives of the owners. And there's also Hugh's room, but where is Hugh?His bedroom is musty and his books and belongings are all covered in dust. Eleanor Nilsson first thought of the story of Gunno and Hugh after she saw an old rambling house near Adelaide. The house seemed welcoming yet elusive and mysterious. This is the house in The House Guest. The dog in the story was inspired by Lochie, Eleanor's own Shetland Sheepdog.