The Last Horcrux/Lestrange Manor/Bathroom/Fantasise about other girls or women
You start to fantasise about the girls - and the women - that you will get to meet at Hogwarts. There are about seven hundred pupils at Hogwarts at any given time, so about one hundred per year group. Between the four houses that makes twenty five or so students per year group. This means you might expect about twelve or thirteen first year girls in your dormitory, and a similar number for every other year group. Eighty or so girls, aged 11 to 18, just in House Slytherin!
You are sure you will end up in Slytherin, but the whole school will be full of girls, quickly turning into women, their ripe young bodies aching for sex, just like your body. You look down at your beautiful young pussy, the delicious smooth curves of your outer lips and the sparse, soft black hairs that mean you are becoming a woman.
You raise your bottom out of the water and slip a finger inside yourself, then another, finding the rougher surface of your G-spot and begin to grind your fingers there. You bring your ankles up to your bottom to keep your pussy above the water and start to rub your clitoris with your other hand. Your pussy is aching got you to find the right rhythm and the water is splashing as your body slowly starts to writhe to the sexual tempo you are creating.