Help the confused plumber complete his text message to his boss.

I'm standing outside ___ school right now, but I can't find the leaking pipe you told me to fix!

The correct answer is the.

When referring to an institution as a specific physical building rather than its primary educational purpose, we use the definite article "the". Because the plumber is there to work on the building—not to attend classes as a student—he is standing outside "the school."