Why is my young daughter so obsessed with ?

My 8 year old daughter keeps getting in trouble for masturbating at school. She is very boy crazy and chases boys at recess to kiss them. I've caught her and the neighbor boys several times showing each other their . She also like to write stories (which are very good for her age!) but some are also pretty graphic and ual in nature. What do you think? I have asked about ual abuse but she is very sure that no one has ever 'touched' her inappropriately. This has been on and off for a couple of years now!