Oh yes, also...
to understand the lack of girls, you have to understand the lead-up to the book. It takes place during the bombings of England. Historically, during this time, boys that attended private schools (known as public schools in England.. weird, I know) were flown out of the country to avoid the dangers of remaining in the country. And yes, those schools were all-boy schools. Ironic, though, isn't it? That when they were flown out of the country it was to protect them? And look what happens...