Let \( x \) be the total number of students at McCord Middle School. Given that 200 students represent 40% of the total student body, we can set up the equation:
\[ 0.40x = 200 \]
To find \( x \), divide both sides of the equation by 0.40:
\[ x = \frac{200}{0.40} = 500 \]
Thus, the total number of students attending McCord Middle School is 500 students.