Ghost Knight

Ghost Knight is an awesome book. Jon Whitecroft, the main character of this book, never expected to enjoy boarding school, but on his first night he sees a bunch of crazy ghosts who call him "Hartgill". Coincidentally that was his mothers maiden name as well. Then he meets a quirky new friend who is called Ella Littlejohn and asks about the ghosts. She says that there is another ghost who can help him. So they have to summon the long dead knight William Longspee but at what cost?
https://www.youtube.com/watch?v=3nZzP35anHE