Emptiness and Silence:
* The empty school was as silent as a tomb.
* The school was as empty as a deserted island.
* The hallways were as quiet as a snowfall.
Lack of Activity:
* The school stood still as a statue.
* The classrooms were as inactive as a dormant volcano.
* The playground was as lifeless as a forgotten graveyard.
Sadness and Loneliness:
* The empty school felt as lonely as a lost puppy.
* The building was as sad as a wilted flower.
* The school was as desolate as a forgotten dream.
The best simile will depend on the specific context and the emotions you want to evoke.