Malia / Isaac
I think they did plan on telling her, but first step was "you're in another world, it's another time, you can't see your family, we're stuck here, also there's a TV show and werewolves." Second step is "and in that show you died." It's okay, dude, everyone knows you didn't mean to freak her out. Just know that she and Derek don't know ... most of what happened. As far as I can tell.