That is kind of odd. Air Conditioners remove the humidity from the air as they are running by water vapor condensing on the coils and then running out the condensation drain line. You should see the water dripping from the line while it is running. Setting the temperature colder while you are gone will definitely help as it will keep the unit running more and condense more moisture and remove it from the air in the room.

The only things I can think of is if someone is leaving the room door open, especially a door to the outside like a balcony and allowing humid air to re-enter the room. Also, and this is likely, they did not properly ventilate the bathroom so when everyone takes a shower, you are introducing a ton of moisture/humidity into the room. Another possibility would be if the unit is too large (too high BTU's) for the room size so it isn't running the compressor enough and cycling in and out too quick. That is doubtful because they would most likely use the lowest BTU, cheapest unit they could get away with.

I would try setting the temperature down around 65 while you are gone for the day and then turn it back up to your comfortable temperature when you return.