jmc

Let the number of boys be $2x$. It follows that the number of girls is $3x$. These two values add up to $30$ students, so $$2x+3x=5x=30\Rightarrow x=6$$ The difference between the number of girls and the number of boys is $3x-2x=x$, which is $6$, so the answer is $\boxed{6}$.