Todd's suggestions are right on, and this can be seen as simply expanding on them: When short on time, benefit as much as you can from other people's effort. Yes, you could do your own analysis from scratch, but you're not the only person to encounter this sonata. Cambridge publishes an entire "handbook" dedicated it, explaining a lot about the background, the reception history, and the actual material ("Appendix II" is "Formal and thematic outline"). The same author, Geoffrey Block, has a chapter on the Concord Sonata in a 1997 "Ives Studies" volume. One more book is dedicated entirely to this sonata, Charles Ives's Concord: Essays After a Sonata (apparently a play on Ives's own Essays Before a Sonata).
Of course you should spend most of your time practicing, but you should sink some time into this research. I hope, at a conservatory, you're familiar with the usual tools for academic research—JSTOR, RILM, etc. If your school library doesn't have the books above, they can probably get them using Interlibrary Loan if you get the request in quickly. For any piece, it's nice to research the background and check out other people's analyses, but it seems vital for this piece. Even a few minutes' Googling suggests it has a rich backstory, programmatic associations, and complicated reception history.
Similarly, I'd double down on Todd's suggestion that you listen to recordings. People disagree about whether they like to do listen to recordings while learning a piece, but whether or not you make a habit of it, it would definitely help jump-start a quick study.
And similarly, get all the help you can from people who have already studied the piece. If your teacher doesn't mind, see if you can get pointers from any other students who have played it.