This happened when I was twelve.
I was twelve and was sitting at the back of the class with a girl with whom I was quite friendly. I was through puberty quite well. I had some nice thick pubic hairs but the best part was my five and a half inch cock.
We were sitting and supposedly watching a video, but it was pretty boring so we started talking. We some how got on to the topic of masturbation. She asked how often I did it which was two to three times a day. She said that she did it about three times a week. She could see that I was getting a hard on in my shorts (boxer-briefs underneath). She then put her hand on my crotch and felt my rock hard cock. She told me it was like her brother's who she had wanked. His was six and a half long and he was 16. She then asked me to do to her what she was doing to me.
She put her hand down and grabbed my cock and started to stroke it slowly. I put my hand under skirt into her panties and she was wet. She had to help me a little bit but I sort of knew what to do from the porn movies my mate and I had watched. I continued to rub her clit while she rubbed my cock.
We continued until she stopped and said she was about to cum and I rubbed quicker and quicker and shoved my finger in and she let out a little moan as she orgasmed. She then continued until I blew my load right into my boxer briefs and onto her hand. She cleaned up with a tissue while I went to the toilet to clean up.
We have remained quite close and have done this a few more times over the years. When I got to high school, some guys were also interested in my cock in the locker room. In case you're wondering, it is know nine inches.