To find the total number of parking spaces in the school, we can use the information given about the handicap spaces.
Let \( x \) represent the total number of parking spaces. According to the problem, 15% of the total parking spaces are handicap spaces, and we know there are 18 handicap spaces. We can set up the equation:
\[ 0.15x = 18 \]
Now, we can solve for \( x \) by dividing both sides of the equation by 0.15:
\[ x = \frac{18}{0.15} \]
Calculating the right side:
\[ x = 120 \]
Therefore, the total number of parking spaces in the school is \( \boxed{120} \).