YA book, sister and brother end up in hell after accident

I read this book in middle school, an older sister who is rebellious shoplifts something from a store in the mall. Puts whatever she stole in her brothers bag, there was an accident and they end up going to hell which is kind of like a school. The younger brother is a “good kid” saying he shouldn’t be there because he didn’t know the stolen item was in his bag, and becomes obsessed with returning to the real world. The principal, who I think is Beelzebub, says mistakes don’t happen, that even Mother Teresa is there, and very few people actually make it to heaven. I think she eventually discovers that his being there is a mistake, but tries to hide it and keep him there.

I remember the cover being greenish black, and I think the brother and sister were on the cover.

I appreciate any responses, thank you!