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.

Check out these sentences:

The new system only confused the PA -It did nothing other than confusing her
Only the new system confused the PA -It's the new system, the old one did not confuse her
The new system confused the only PA -there is only one PA in the office

In some way, only and all in this context are the same. I mean, the meaning of the sentence changes depending upon the placement of those magical words. Here, these words apply to the word following it. This is quite clear from the above mentioned sentences.

A similar magic word 'all' in your question also works in the same way. It depends on the following word that comes after it. And, it changes the sentences accordingly. I may not agree with Pyraminx that only the fourth one is acceptable. To me, all sentences make sense. It's just the placement of the word 'all' (like 'only') that confuses us. I think you have pretty well understood the nuance.

All my friends answered my questions in the class yesterday -'All' followed by my friends so you are emphasizing on "every one of my friends answered my questions..."

My friends all answered my questions in the class yesterday -'All' followed by 'answered' which means the emphasis is on everyone answered your questions...

My friends answered all my questions in the class yesterday -'All' followed by your questions that means they might not have answered anyone else's questions.

My friends answered my questions all in the class yesterday -'All' followed by in the class. This sounds okay as well. They answered all in the class, not outside!

And finally...

My friends answered my questions in the class all yesterday - 'All' followed by 'yesterday'. As you guessed it correctly, your friends answered your questions all day, yesterday.

The position of all in a sentence largely infers what 'All' of something you have.

All my friends answered my questions in the class yesterday.

All the friends you have, in your class, answered your questions.

My friends all answered my questions in the class yesterday.

Some friends of yours (not necessarily all of them, but multiple), answer all the the questions you asked.

My friends answered my questions all in the class yesterday.

This sentence is incorrect/awkward English. The 'all' is largely unnecessary here as it doesn't pertain to anything. It's confusing two sentences, "My friends were all in the class yesterday" and "My friends answered my questions". It would make more sense as "My friends answered my questions, all of which were in the class yesterday."

My friends answered my questions in the class all yesterday.

This is poor English, it's passable in a colloquial setting. It should be 'all day' or 'all of'.