Carry them home to be laundered on Friday. To avoid getting hot, take some extra time and bike slowly. Or ride in early, before the day has had time to heat up. You'll get fit. You might even lose weight.
Aerobic exercise is the way to get thinner, and biking is a great aerobic exercise.
You'll probably feel happier. Regular exercise can often help people manage stress better. Exposure to more sun will also cheer you up if you're subject to Seasonal Affective Disorder. You'll be lighter on the environment. Car engines pollute more than your engine does.