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.