...and by Demon Cross really means Mean and Scary. There are no actual demonic entities, and no souls are eternally damned in the making of this book.
In fact, it isn't even very scary -- rather unrealistically so, given the premise. I mean, imagine you are little kid at a new school. Your new foster brothers drop ominous hints and are clearly afraid of the headmaster and other students. On your first day, all the other kids are strangely regimented and won't speak to you. You get physically manhandled by the precepts, and then dragged to the head's office. Afterward you can't remember what happened. Likewise, think about life for the two boys: every day they go to a school where they are shut in a room while all the other zombie-like children conduct some secret activities. The slightest misbehavior leads to physical abuse, such as being forced out into the snow for an extended period with no coat or gloves. Their parents absolutely refuse to listen and insist the school is wonderful.
The scenario is nightmarish. In execution, it comes across as merely stressful and slightly scary. In real life a child in these conditions would probably have a break-down or at least display some weird behavioral disorders. But I guess they are English or something so they are keeping their chins up or whatever. There isn't enough character development to be certain.
Anyway, it is a very short and slight book, an entertaining quick read that doesn't contain anything graphic or too frightening for younger kids.