A one-room schoolhouse is a bathroom in a commercial establishment where the sinks, toilets, and urinals (if applicable) are in the same room, not separated by partitions.
The term comes from schools from the early 20th century which were the same way -- one room was the entire building, all grades of student in together.
A: "Let's go to Hook's restraunt."
B: "No, I hate that place. Whenever I have to go to the bathroom, it's a one-room schoolhouse. Someone will be in there and either he doesn't lock the door and I end up walking in on him and usually see him naked or the person does lock the door and I'm stuck awkwardly waiting in the way of the servers leaving the kitchen in full view of the restraunt."