I have read multiple WWI era books involving SPIES at a SCHOOL - well, okay, now that I think of it, maybe just two, but that still seems like more spies at schools than anyone would expect. Neither one of them answered this in a really satisfactory manner, as the real answer is "the author wanted to do a cool topical storyline involving spies and it had to fit in with their existing series and/or School Story schtick."

In the story, Chrissie's main goal is to free her brother (a German POW) from a nearby POW camp, which sort of makes sense.... except he doesn't arrive at that POW camp till halfway through the school year and how did she know beforehand that he was going to end up in THAT CAMP?