A small towel comes in handy in class not only during perspiration.
1. Use it as a prop (if you do not have a belt).
Paschimottanasana (Seated Forward Bend)
Gomukhasana (Cow Face Pose) - Hand
2. Use it as padding for knees.
3. Use it as blind during Shavasana, so you don't squeeze your eye lids to close your eyes.