Actually, the fourth is acceptable.
The first, second, third, and fifth make perfect sense, as you've described in italics,
but the fourth is actually acceptable english.
The first one implies that each and every one of your friends went and answered your questions, whereas the second implies that a group of friends are present, and that they are all answering your questions. The third is exactly as you've described.
The fourth one is acceptable english, and it implies that your friends (that are present) answered your questions in and only in the class today. For example,
I ate my cookies all in one hour.
means that I ate my cookies in not two or three hours, but in one hour. This implies that you ate quickly, finishing them in one hour and not anything else.
I ate my cookies in one hour.
This sentence, on the other and, doesn't imply anything, it just tells the reader that I ate all in one hour, not mentioning anything about the speed that you ate it.
so the fourth sentence:
My friends answered my questions all in the class yesterday.
implies that your friends answered your questions in only the class yesterday, and not the class 2 days ago, or the class today.
Whenever you put 'all' between the object and 'in,' it will always allow the reader to infer that the action being done by the subject is hard to accomplish in the given time.