As all good offices should, Harper's office has signs* on the door:
If you click on that photo above, you can probably read some of the signs, but just in case you are having trouble doing so, here is a closer look at my favorite:
I'll translate, just in case your kindergarten spelling is rusty, "Yell in room before entering if no yell back I am not in."
There you go.
*Tonight, before bed, Harper added a not-originally-pictured sign. It says, "sleepping". I imagine she'll take that one down in the morning.