I'm out of college now, but I can provide some moral support since I had something like this happen to me at work the other night. I was closing the bar I work at and counting up tips to divide between myself and the other bartenders. Everyone, including my boss, who locked himself in the back office to finish paperwork, was in a rush to get out early that night, and, as a result, the doors had mistakenly been left unlocked, and I was the only employee still doing closing duties in the front of the bar.
Long story short, some random weirdo walked in carrying on as if there were some emergency outside that needed my urgent attention. Exhausted and not thinking straight, I got fooled by his diversion and thought he might be serious, the ultimate result being that I turned my back to the pile of money that I had been sorting out, and the guy was able to pocket a large amount of cash without my noticing. After the guy left, I went back to finish handling the money and found out that he walked out with over $600. I couldn't just act like the money never existed either, since doing so would have meant my co-workers wouldn't get paid for their work. So not only did I make nothing for my labor that night, I had to make up the stolen money out of pocket (my boss was sympathetic enough to offer to split it with me, but that still means I'm out $300). On top of the financial loss, there's still the frustration and humiliation of realizing I'd been fooled, and the general fear that my competence and honesty (I was alone and had no witnesses to back me up, and the bar industry is notoriously plagued with employee dishonesty and theft) will now be questioned at work or that something similarly bad will happen the next time I close the bar.
But I think the point is that our situations teach a common set of lessons:
1. Losing money sucks, but it's not the end of the world. $100, let alone $300, is still a lot to me, but the reality is that a lot of unplanned and uncontrollable problems will consistently come up and impose costs upon you (e.g. getting a ticket, damaging your car, losing your wallet, etc). You have to realize that these things happen and be grateful that it's on a small enough scale that you can recover without too much difficulty. In my case, I've realized that things could have been much worse had I caught the guy in the act and he turned out to be armed or ready to use violence.
2. As much as you don't want to hear it, security precautions are important. My boss has been kicking himself for forgetting to lock the door right after closing; if the door were locked, the theft wouldn't have happened. The lesson in your situation is not to take security for granted and to hide your valuables.
3. This is one of the more depressing points to take in, but it's important to recognize that people do shitty things and be wary. Obviously, this doesn't mean you should immediately assume your roommate guilty and accuse him (well, unless you try Bandwagon's setup and it works as expected), but this ties in to point 2 in that you should assume that at least some of the people in your surroundings are capable of this behavior and take precautions to prevent theft, considering that you're in a communal setting with limited private space.
4. Letting a bad experience like this control you will make things worse before they make them better. I'm probably going to go apologize to most of my co-workers at work tomorrow, since I was so eaten up by the whole ordeal that I basically crawled up into a shell at work yesterday and blew everyone off. You mentioned having a math test to deal with; letting the theft influence your studying and performance on the test would mean you would be allowing the theft to magnify the damage done.
Hope that helps somewhat.