Post History

I endorse Evil Sparrow's answer.  If, however you must lead with the old man for some reason, you can return from the room back to him with a paragraph break.  Paragraph breaks are (small) discontinuities and you should use them when changing subjects, just as you do when changing speakers in dialogue.

> The old man gave a lecture in front of the class. The class was a really small classroom with a handful of seats, the windows gave a view, a nice view of the outside, and illuminated the classroom, which had a depressing atmosphere to say the least, almost creepy in a way. The desks were very old and stank of mice and burning. 

> The old man had a very strange demeanor as he was talking. He held his chin high and sometimes would find himself lost in thought, which made the students wonder if all his head was in there.

The first paragraph here establishes the lecture setting (and the old man) in the first sentence and transitions into a description of the room.  That all fits together because the old man *is part of* the setting here.  The paragraph is primarily about the class.  The second paragraph is, instead, about the old man; even when it mentions the students' reaction, it's primarily about him and his mental state.