Emphasizing emptiness:
* The empty classroom was as quiet as a tomb.
* The room was as bare as a freshly painted wall.
* The desks were as still as statues.
Emphasizing silence:
* The empty classroom was as silent as a snowflake falling on a fresh blanket of snow.
* The silence in the empty classroom was as thick as fog.
* The room was as quiet as a library after closing time.
Emphasizing stillness:
* The empty classroom was as still as a photograph.
* The room was as motionless as a sleeping giant.
* The chairs were as unmoving as rocks.
Emphasizing loneliness:
* The empty classroom was as lonely as a lighthouse keeper on a stormy night.
* The room was as forlorn as a forgotten toy.
* The classroom was as desolate as a ghost town.
You can choose the simile that best fits the context of your writing.