I'm 15 and I go without panties fairly often. When I was in 6th grade, we were outside doing track & field stuff for gym class. Near the end of class we went to sit on the bleachers, and my friend Sarah sat right in a puddle of rain water. Her gym shorts were totally soaked! She was really embarrassed because her shorts were now transparent in the back, and you could see her panties.
After class we changed back into our school clothes, but her panties were still too wet to wear. So she had to go the rest of the day with no panties on. At least her skirt wasn't too short. It was a few inches above her knees.
I was teasing her about it a few weeks later, and so she dared me to go without panties for a day. I said I would only if she did too. So the next morning we took off our panties before homeroom, and went the whole day without them. I was really nervous, and it felt sooooo weird!
We tried it again a few times, with shorter skirts, and then started doing it more often. Ever since then I only wear panties about half the time.