After her battle with Torchwick, Ruby is arrested by Goodwitch and brought to a Police station, where she meets Professor Ozpin of the Beacon Huntsmen academy, who immediately takes note of her silver eyes. After discussing how Ruby is training at Signal to apply to Beacon and how she wants to become a Huntress, Ozpin lets her enter Beacon 2 years early.
This scene effectively boils down to Ruby being given the choice of enlisting or doing prison time. She acted in self-defence, and stopped a robbery. Get a lawyer, Ruby.
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