Do you lose weight from walking from the third floor to the first floor at school between classes?

When you walk, even when going downhill or down the stairs, you are burning calories. Any calories you burn come from the glycogen stored in your body in the form of adipose (fat) tissue. So, even if you are not losing weight, you are doing what you can not to gain more. Walking up the stairs burns more calories than walking down, so avoid the elevator.