Most Helpful Girls
Have horny thoughts, swim in the nude (if you have a pool or ocean/lake nearby, take a shower or bath, etc.
You haven't gotten into the pillow humping thing? Get a clean washed pillow with a pillowcase and put it between your legs, lightly rub yourself against it. The corded pressurized showerhead works, as well as the faucet if you can get close enough to it. Those work if you don't want to touch yourself. The easiest way to get yourself all wet is erotic literature, photos, or of course, porn. At your age, I would stick to literature though.
Sometimes the thought of a classmate you are attracted to will do it.