Missy is right. You should not hide your birthmark just to make yourself suffer in the spring or summer. I have a facial PWS so it would be impossible to hide my birthmark. You will get looks and every once and a while get questions about it, but you they can tell it bothers you then others will make a bill deal out of it. My fiancee and my friends almost forget I even have a birthmark even though you can almost see it from a mile away. LOL
I learned at a very young age to just let the comments go in one ear and out the other or to let them roll off your shoulders. Some times I even look at it this way they are just making a fool out of themselves. I work in retail and everyday have people mostly children look or point at my birthmark. Just make it positive, and educate the children then it makes you feel like you are doing good for the public and children.
Stay strong and do not let anybody hold you back from what you want to do in life even if that is wearing shorts and skirts.