I was walking home from school with a girl in my class who's very open about her bathroom habits. She kept farting and she said she was busting for a poo and if she didn't find a bush very soon she was going to poo herself. It really turned me on and I had a hard on which she soon noticed. We found some bushes about a hundred yards away and Julie took me into the undergrowth with her and she lifted her skirt up and took her knickers off and handed them to me. I they were wet and she said she was really horny. She squatted in front of and began to poo as she said I could wank in her knickers if I wanted. I needed no encouragement and unzipped my penis and wrapped the crotch around it so I could feel her wetness as I masturbated. I waited until she finished her poop before I came into her knickers and man it felt so good. Julie stood up and let her skirt fall back into place and I asked her if she was going to wipe but she said she had nothing to wipe with and she would wipe her bum when she got home.