You need to answer this question: what
need does contributing a review solve for the reviewer? What do they get out of it?
StackOverflow bet big, and won, that technically inclined people would contribute millions of dollars of free consulting if it awarded them with, essentially, the geek equivalent of WoW loot. (Pixels of no importance or cost outside the game but, critically, important to the players.)
HackerNews uses the karma system. I probably write too much, but one reason I write is that there is a little scrolling You Helped Somebody Out Today counter in the top right corner, and that gives me warm fuzzies and a sense of social worth.
If your site has chicken/egg dynamics to it, your content acquisition strategy should be based on giving people reasons to donate you chickens in the absence of eggs or eggs in the absence of chickens. I love Delicious as an example of this: if you're the only user of Delicious, it still has value, because it was a very good multi-PC bookmark manager when that feature was underserved.
You can also seed content, which can be quite cheap if you do it intelligently. For example, I assume you put up pages for classes which you don't have reviews for, right? Good. Rank order those by access and start at the top. Pay people to write seed reviews for the content most in demand. College students' time is ridiculously cheap (an insight which founded the entire discipline of behavioral economics). You can probably get 5 reviews for pizza money.
For the last several years I've paid for content for my website. At the start, when I had no idea what would be popular, I just used my intuition and aimed at obvious high-value targets. As soon as you do that, you'll start to collect Actual Data From Users. Adjust aim, fire again. The beautiful thing about evergreen content (I'm not sure yours is timeless, incidentally, which worries me) is that after you acquire it you get to keep the benefits from it for forever. This means that if you happen to realize "Oh, shoot, I was in a local maxima" that isn't a problem, you just expand away from it. (This has happened to me multiple times and is a story for another day.)
You have other problems with your website in terms of conversion which are bouncing these users, incidentally.
1) The design wastes far too much space and does not immediately communicate a value proposition. I would usually demonstrate this with a color-coded screenshot but I'm on Linux right now and refuse to fight GIMP to do it: less than 5% of the pixels on my monitor display anything related to textbooks when I open up your site. I don't know what the magic right number is, but it sure isn't 2) Your site cares far too much about its needs and not enough about the user's needs. Thinking this is not a major sin. Telling the user about it is. "Running a site like this requires a lot of help from students like you" is a bad tact, because it requires people to give to you before you give to them. Most Internet uses are quite keenly interested in What Is In This For Me.
3) Surfacing reviews on the front page is a great way to point out both the value of the service, to establish social proof (nobody wants to go to a bar where no one drinks, nobody wants to post the first piece of content to Empty Social News Service #432), and to reward power users for their dedication. It is frightening how much you can get out of people just to give them a chance at community recognition.
4) "To get started, just click “Find Your Course“." I think there are very few total absolutes in web design, but this is one of them: if you ever find yourself telling people where to click on a web page, and that is not "Click here", you're wasting conversions to no benefit to yourself. You've got hyperlinks. Use them.