Or more accurate, perhaps her immune system didn't work. It may also be possible (I didn't read the full story) that she was exposed to the real flu too soon after receiving the vaccine. If that happened, her body may not have had time to fully develop antibodies that could fight off the flu.
I’d be surprised if she didn’t get the flu shot at the beginning of the school year. Most states that require the shot for schoolkids require it by the start of school or shortly thereafter.
I’d be shocked if she received it after Halloween even.