jail: A place of confinement for individuals who have been convicted of a crime, usually a more severe form of punishment than other options.
That's a bit stiff: This phrase suggests that the idea or statement is a bit too harsh or severe. In this context, Mary is expressing her opinion that sending someone to jail might be too severe a punishment.
lying around: If something is "lying around," it means it's left in a place without being properly stored or secured. In this context, Rachel is questioning whether Jaz left his phone unattended.
wrongdoers: Individuals who have committed a wrong or engaged in inappropriate or unlawful behavior. Mary uses this term to refer to those who commit crimes.
I'm all for: This phrase is an expression of strong support or agreement. In this context, Jaz is expressing strong support for expulsion or a long suspension, indicating a preference for such consequences.