This novel suggests that the reader take a closer look at language. The premise mirrors the premise of one of the most popular comic book franchises in human history, X-Men, which basically does the same thing as the mysterious school in this book, taking disenfranchised but powerful youths and training them in their supernatural powers to become forces of good. However, in this book, Emily's superpower is just language.
This means that language is a kind of supernatural, magical power, which might explain why the title is Lexicon. The word lexicon has two meanings. A lexicon can mean all the words that a book contains, or it can mean all the words that a person knows. In this book, it means both, because Emily has to learn words and their meaning to exploit their magic power.
The question of persuasion is therefore a game, where Emily has to use the precise word that connotes and denotes what she wants, but if she can find the "magic words," so to speak, she can often get what she wants. This is hinted at even before her admission to the school, because she has to find a way to survive, and she hustles people on the street. That is a picture of her using her natural abilities to get what she wants, which is what the school amplifies. The main way of getting what she wants is clearly to say the right thing at the right time.
