Thanks for your response. What you say about fraud does make sense, though I disagree with your comparison of this situation to booking a seat with extra legroom. In that case, the school incurs an excess expense on my behalf, which is not the case here. For instance, School B did offer to book and reimburse a roundtrip from my home to the school. This would have left School A's flight in place. I don't see how it would make it any better if I chose to do this, when I knew there would be a cheaper alternative for School B (they save one leg of the trip essentially) that would not require any extra funding from School A. So theoretically, I would be saving academic funds across departments with this rearrangement. Anyway, I agree that it would be good to speak with the school in question about this, and will do so soon. No point in stressing over it really.
On another note, I must say that United's policy is terrible. I called today (to the tone of a rather unfriendly agent), and essentially I incure a penalty of $200 for EACH LEG of the flight I modify. Canceling leads to a credit of the price I paid minus $200, which is ridiculously high. From what I've read online, it does seem that a good number of people chose to forfeit their money instead of giving United the opportunity to make even more money from their ticket via resell. Maybe I'm just spolied by the reasonable policies of Southwest, but I will try to avoid United Airlines for future travel.