Answer: Conventional
Vandalism is a offense which involves the deliberate destruction of public or private property. Conventional reasoning refers to the ability of adolescents and adults to justify the actions of other human beings with respect to the society ethics and expectations.
Joseph is at conventional level of moral reasoning in which he is thinking about the fact that vandalism is an offense, which should be stopped in the school in order to maintain the reputation of the school. His concern is for the society.