Be BOO to your school.
Cordelia Liu moves from California to the Northeast because her father has lost his job. She enrolls in the local school only to find out that it is a quirky building that is haunted! She does meet a friend, Agnes, but when the ghost sightings start piling up, she also gets to know Benji. Benji was a popular kid and great soccer player until he started to hide behind his hoodie and earbuds. Cordelia suspects he can also see the ghosts, and she's right. He introduces her to the vast variety of characters who occupy their school, and the two try to find a way to deal with the problem. The school's history is rather dark, and the founder, Elijah Shadow, met an unfortunate end, so getting to the bottom of the problem isn't easy. They eventually figure out "brightkeys"; objects the ghosts seem to need before they move on, and they manage to send some of the ghosts to a happier place. Their principal, Dr. Roqueni, isn't happy with this, especially when poltergeists who trash the cafeteria are stirred up. Shadow was her relative, and she has taken over the principalship to preserve her family legacy. She thinks the children have riled the ghost hunters, who clear the building of ghosts, and wants them to stop. Cordelia thinks that ghost hunters kill the ghosts. With the help of Agnes, who can't see the ghosts, the children try to solve the mystery of the building, the ghosts, and the awful legacy of Elijah Shadow.
Agnes is a bit odd, but it's reassuring that she reaches out to Cordelia and the two become friends. Benji's transformation (the earbuds block the noise of the ghosts) also makes sense, and I'm sure he is glad to have help in dealing with the paranormal. There is some cultural diversity, with Cordelia's father being Chinese, Benji being Latinx, and the Shadows being African American. There are a number of teachers and staff members who play a role in the mystery, and while some are rather extreme, most are realistic and add a layer of believability to the story. The twist at the end with Mr. Derleth is rather sweet, and I see him being helpful to the children in the next installment of the series.
The reasons for the ghosts infesting the school are well explained, and the history of the Shadow Family is intriguing. This reminded me very strongly of the work of Dan Poblocki, Betty Ren Wright, West's The Shadows or Nielsen's Dr. Fell and the Playground of Doom. It isn't quite as dark and scary as this author's Nightbooks, but is an appropriate level of scary for middle grade readers.