Next fall, a stunning $55 million high school will open on the edge of Fairmount Park here. For now, it is called the School of the Future, a state-of-the-art building with features like a Web design laboratory and a green roof that incorporates a storm-water management system. But it may turn out to be the school of the future in another sense, too: It is a public school being used to raise a lot of private money.