It happened so suddenly, and quietly, but it happened all the same: everyone in the Common Room, whether they had been lounging there for hours or had just walked in the door, was suddenly evicted.

An unseen force repelled all Slytherin students from their Common Room and attached areas. The same force locked all entrances and exits to the rooms with one resounding, echoing, ominous CLICK.

The Common Room was officially locked and sealed to all.