I have been working on this report all morning
does NOT mean that you are necessarily working on it at the very moment of utterance. In fact, if you are speaking, you cannot really be working on the report. And as R. Sole said in the initial comment, "continuous" in a grammatical sense does not mean the same as "continuous" in the mathematical sense. It does not mean that you refused to answer your boss's phone call. It means that you worked on it the great majority of the time.
The temporal rules about tenses need to be interpreted in a socially relative sense, not according to the rules of classical physics. The present perfect can be used to speak about actions or states in the relatively recent past.
You may say
I have been working on this report all morning
as you eat a hot dog at your desk at 12.11 p.m. It is recent enough to count as what your teacher is calling "NOW." Of course, you could also say
I was working on this report all morning
The two statements would have the same meaning in that context. But you cannot say as you leave for the day at 5:11 p.m.
I have been working on this report all morning
That is not relatively recent enough to count as "NOW." It is not idiomatic. Now you would have to say
I was working on this report all morning
When you added your second sentence about completing the report, that changes the nature of the case. Usually, the continuous implies that something is not finished, but you are qualifying that meaning. In the situation that you outlined, where you finished it two minutes ago, the use of present perfect continuous seems to me idiomatic. Two minutes ago is still socially "now" to me. So I disagree with R. Sole, but notice that he did not make an absolute comment; he said what he thought was "more likely" so my disagreement is far from profound.
I cannot quote authority for what I am about to say. It comes from the experience of having written a great deal over the last fifty-five years. My impression is that when you qualify the use of the present perfect continuous as you did, what is socially "now" shrinks considerably.
I have been working on this report all morning. I finally submitted it an hour ago
sounds bizarre to me. I would definitely say
I was working on this report all morning. I finally submitted it an hour ago.
Once you think about natural languages having a social context, ideas like "now" take on a somewhat fuzzy meaning and lead to minor disagreements such as the one I have with R. Sole. I would have no trouble saying
I have been working on this report all morning, but hit the submit button less than a minute ago.
In short, "now" is not to be interpreted with regards to English tense structure as meaning an instant of infinitesimal duration. It is a period of finite but relatively brief duration with the brevity varying depending on social context. Similarly, "recent past" depends on social context. English tense structure can convey subtle degrees of temporal nuance, but it is always fuzzy along the edges because it is defined socially rather than physically.
