My experience is that the iPhone can drain serious amounts of battery power if it's constantly looking for a signal and not finding one ...

Here's my theory: when you're on the bus, I'd wager you're going through areas of spotty cellular coverage and the phone is using up battery energy looking for something to lock onto. I'm going to guess coverage at school and at home is better and that's why the battery usage stabilizes at those locations. That would also account for why it stabilizes when on the bus in Airplane mode, too =)

When riding the bus next time, try to notice if you get a lot of weak signals or "no service" indicators. If so, then that's probably the culprit ...