I shared this story in the whale tail stories thread but I figured it would fit well here too. Hopefully this is what you are looking for.
I had PE class one year with this girl Ashley. She was really cute with dirty blonde hair and a slim figure, she was even a cheerleader. For some reason she never wore her PE uniform to class and just wore her regular clothes, I never knew why. Anyway, one day she has on a t-shirt that comes to about her waist and a pair of black low-rise jeans that I think were from American Eagle or Hollister. She was sitting in front of me at the beginning of class when the teacher was going over instructions. I of course look down towards her ass since I was always on thong watch back then lol. I notice that her light pink Victoria's Secret classic v-string was showing a good 2 or 3 inches out of her jeans. Of course, my heart was racing at seeing such a nice whale tail on a hot girl. A few seconds later, I notice her reaching around and feeling that her thong was out. She then frantically tries to tug her pants up to cover it. Of course, since she was sitting down and she was wearing low-rise jeans that were a little big at the waist, this was difficult to do. So she tugs her pants up a little and her thong is covered but then it slips out again and she eventually reaches around and struggles to tug her pants up again. This happens at least 3 or 4 more times, she really wanted to keep her pants up but just couldn't lol.
So later on in the class we are split into groups to do various activities. I end up in her group and we are at this station where we are supposed to jump rope. Two people would hold each end of the rope and another person would jump the rope. She starts out jumping the rope and I am at one of the ends turning the rope. She jumps for a while and keeps stopping periodically to pull her pants up. She then eventually gets frustrated and says "can someone switch me, my pants keep falling down". Being a gentleman lol, I tell her that I'll switch her spots. I walk over to wear she was at and she walks over to my end of the rope. As she bends down to pick up the rope, I notice that she has about 5 or 6 inches of her pink thong showing. Her pants were literally about halfway down her ass so I even saw the pink string disappearing between her cute, pale little butt cheeks. My heart was obviously pumping after getting such a show. Luckily, I don't think she noticed me looking any of the times but I will definitely remember that day.