You may well lose grades since you didn't complete the work but I have two anecdotes that may help you out.
Number 1:
On one project I was involved in, we missed the deadline slightly and the quality was not up to par (that was the real issue, missing the deadline was a minor point).
We were all worried about facing the board of the company (this was a high profile project) and didn't know what the outcome would be.
We went in and our boss opened with the line, "We screwed up"
which was, we thought, an interesting approach to take :-)
However, he basically went on to explain that it was our fault, that we had committed to the deadline and quality targets, and that we weren't up to the task.
The next phase of his presentation was detailing how we planned to extricate ourselves and ensure that delivery of phase 2 was going to be successful. Then an understanding of any decision the board would take regarding cancellation of the project or disciplinary action.
Basically offering our heads up to the chopping block, so to speak.
I think that the admission of guilt was basically what won the board over. When you accept responsibility for a screw-up, there's a primeval protective need in most people to say it's not as bad as you think. Or at least it certainly put them off their guard.
Number 2:
This actually was a friend of mine whose task for a third-year university project was to build a screen editor for the VAX 11/780 (this was back in the mid '80s). Because there was a wide range of terminals, the University wanted a solution that would work for them all.
It wasn't finished on time but, because my friend, clearly stated what was done and what had yet to be done, they actually got fairly good marks. They weren't developing a product for sale, this was something that could be used for the university but its primary purpose was the education of my friend.
And, it gave them a project to give to someone the following year, thus lessening the load of the lecturers in coming up with more ideas :-)
Again, I think simply because there was an admission of guilt or failure, the examining panel was willing to let it slide.
Summary:
You will find many similar situations in a real work life, real power will lie in adaptability (both yours and the company you're working for). That's because you can't foresee everything.
As an aside, the year following my friend's project, they tossed out all the annoying terminals and standardised on DEC VT100 and better terminals, so EVE (the Extensible VAX Editor) worked fine everywhere. I don't think that project was ever picked up and finished.
That will no doubt happen to you quite a bit in the real world as well (projects that you've poured your heart and soul into, being tossed on the scrap heap).
Don't say the deadline was unrealistic, that is an attack on somebody else and unlikely to endear you to them (or the panel).
You could be somewhat more tactful, such as "I didn't foresee the difficulty of the work when I took it on" and put in place a plan for future work (on the project, not something you have to do). Accepting the responsibility will invoke that sympathetic nature in your panel.
This is Social Engineering 101 (which is rarely taught in our institutions, just something you pick up after a decade or two of dealing with people).
Best of luck.