Teach where?

I had to fill in for Part Time Colleague at 9am today, due to the fact that he was on holiday, so I prepared my lesson and then asked Old Secretary which room the student would be in (it was a single, intensive class). She checked the room list and said, 'Room 18.' So, I put my materials in room 18 and waited....and waited....and waited. At 9:10am, I went back to the office, confirmed that she had said room 18 and then waited in the office (the student would have to pass the office on his way to room 18). At 9:15am, Young Secretary rushed into the office and asked, 'Why aren't you teaching?' to which I replied, 'My student isn't here yet.' She then said, 'He's waiting in room 24 for you.' I let her know that OS had told me room 18, we both gave her a disapproving look, I gathered my materials from room 18 and dashed up the stairs to room 24.