What I find interesting is that a 12 year old Nicola could just about get away with the communication cord, getting out of the train and into the tunnel etc. Much older and it would have been considered far more serious - as Rowan pointed out, she might have caused a major accident. A child reading this would have been completely on Nicola's side and would not see the rescuing of the knife as implausible or that there was any alternative or that Nicola was anything but rather dashing (or so I imagine - certainly I went along with it all whole heartedly when I first read it).

Looking at it from an adult point of view I can still see that an impulsive 12 year old (in similar circumstances) might act thus and be forgiven. An impulsive 14 year old (Ginty, for instance!) might very easily be expelled for a similar action.