Book where an elementary student and his younger sister die then go to a school for dead children

[Sophia]

The book is a children’s book, and I read it around 8 years ago. I remember the first character we follow had glasses, and the colors of the book were more grayish blues. It was also an ebook and a comic on an app I don’t remember the name of. It’s been a while so I don’t remember much sadly.

[Zev]

This might be a long shot, but could it be "Olivia kidney and the Exit Academy" by Ellen Potter?

[Kris]

Link for Zev's suggestion - Olivia Kidney and the Exit Academy by Ellen Potter.