If there are 240 girls, find the number of boys in the school.

If $\frac{5}{8}$ of the students are boys, then the ratio of girls is $1-\frac{5}{8}$, that is, $\frac{3}{8}$.

Now, let $x$ be the total number of students.

Thus, we have:

$\frac{3}{8} x=240$

$\Rightarrow x=240 \div \frac{3}{8}$

$=240 \times \frac{8}{3}$

$=\frac{240}{1} \times \frac{8}{3}$

$=\frac{240 \times 8}{1 \times 3}$

$=\frac{1920}{3}$

$=640$

Hence, the total number of students is 640.

Now,

Number of boys = Total number of students - Number of girls

$=640-240$

$=400$

Hence, the number of boys is 400.