I too, commute on my road bike, 1-2 days a week, for about 6 weeks out of the year. My commute is 14.5 miles one way and I also bring my clothes in on Monday for whatever day I'll need them. I usually leave a jacket and a pair of flats there until the end of the school year, so I have part of an outfit there all of the time. I also have all of my hygiene supplies there, too. I ride with a Camelbak with the bladder taken out, so I can bring some make up, lunch, and my wallet, glasses, and a few papers if I need to. Sometimes I bring a couple of lunches to school at a time, so I don't need to carry that.
I had all visions of riding my Jamis Coda to school, with the pannier, but it has ended up being my around town, errand bike. It's just too hilly here to ride the Jamis 15 miles and get to where I need to be in an hour. I already leave the minute the sun comes up, so I am at school before any kids arrive, and most of the teachers, too. That way, I have time to clean up, change, and get work done before the day really starts. I am planning to start commuting this week (if the weather is OK) and I think I will actually take a longer way home, that includes a nice downhill instead one of the two longer climbs on my usual route.
I've thought about a messenger bag, but my balance isn't that great to begin with, so I think I will leave things the way they are.