Personally, [And I may be wrong] my knowledge is that XNA and Macs, even Mac friendly applications, don't get along with each other as seamlessly and efficiently as borrowing a old Windows computer would be, especially since your professor is almost certainly going to be assuming you are using a Windows computer. Using a Mac may have unfortunate side effects that could lower your grade. The likelihood of this isn't guaranteed, but I would personally not take it.

Unity is [usually] used for 3D game design, so I would use Xam, as it is more streamlined and efficient for your purposes, which is making smartphone games.

And yes, Xam does run on Macs, thankfully.

I see no reason on why that wouldn't work, however, I would ask your professor to make sure he's cool with it.
His opinion matters far more than mine, on that matter.

You can just install windows on your MacBook as well as OSX. Then you can use OSX for your personal stuff and Windows for your homework.

^Thats true, using bootcamp or the like for running Windows. I hadn't thought of that.
Heres a link.
http://support.apple.com/kb/ht1461

http://developer.xamarin.com/guides/

Good luck.

Microsoft used to come around University telling the students that XNA is deprecated in favour of MonoGame (an open-source XNA clone). The VisualStudio IDE used to crash every time they did a demonstration of the content pipeline with MonoGame

http://www.monogame.net

At risk of getting a lower grade because of minor differences between the two, I can't suggest that you use it. But you should probably evaluate it yourself and make the choice.

Your tutor might respect the fact that you are looking outside the box and towards solving issues of portability (a major topic within the game development industry).

If you are stuck with C#, an option that I would personally suggest is OpenTK. It is basically a very portable (and pretty thin) .NET binding around OpenGL, OpenAL etc. Anything you learn here will almost directly be translatable in later life to C, C++ or other languages using OpenGL. Including Javascript and WebGL (or preferably Emscripten).
For 2D you should find it pretty easy. For 3D naturally it becomes a more steep learning curve because you will be learning the fundamentals of modern 3D graphics programming rather than just scripting and automating products like Unity.

http://www.opentk.com