Hello,
Doesn't he know that you are at the school?
Does he not know that you are at the school?
NOT A TEACHER
(1) You have asked a great question.
(2) Here is what Mr. Louis Alexander says in his popular Longman English
Grammar (1988 edition, page 255):
We use the full form in formal questions or when we require special emphasis to express anger, surprise, etc.:
Have I not asked you again and again to be here on time?
Mr. Alexander adds that we also may use it for rhetorical questions (when we are NOT expecting an answer from the the other person):
Are there not more than enough weapons of destruction on earth?
(3) Let's look at your sentences.
(a) Maybe (maybe!) "Doesn't he know that you are at school?" is a genuine request for information. There is the possibility that he does not know -- for whatever reason.
(b) Maybe (maybe!) "Does he not know that you are at school?" shows a note of anger or surprise (depending, again, on the situation). In other words, he SHOULD know.
