Does this sound too rude?
Dear (faculty): As the end of the semester approaches, I am becoming more and more busy. Please inform me as far in advance as possible when you are going to send a student to talk to me for any reason. There is no guarantee that I will be in my office or able to take on the task you ask me at that moment, especially if it involves tutoring or administering a test to a student. I would be happy to make alternate arrangements for the student if you tell me far enough in advance, but am unable to accommodate their needs if you tell me on such short notice. I would also like to remind you that I do not have any permission-granting abilities, and sending the student to (supervisor), (accountant) or (registrar) would be more appropriate when they have a question concerning financing, switching a test date, needing extra time on a test, etc.
Also, if you need to order anything externally, such as food or drinks, I would appreciate if you would tell me at least two days in advance in the future.
LACK OF PREPAREDNESS ON YOUR PART DOES NOT CONSTITUTE AN EMERGENCY ON MINE.