Oh come on. It's obvious "why" there are centuries old vampires in high school. It's just wish fulfilment from the author who wants her favourite soul mate to land right in the middle of her life circumstances and pick her up from there. She wants to be rescued from her high school life that is filled with people she can't relate to or connect with, by her ideal partner, her soul mate, who by "struck of luck", lands exactly in the same setting she's in and is everything she's always wanted from life, everything her unfulfilling and unsatisfying life pushed her to daydream about. In a vampire filled universe, only someone on a school marathon or someone on a bet would want to return to high school. There's also the possibility that a vampire turned long ago enough would want to experience 21st century high school. But since neither are Edward's case, it's just obvious it's wish fulfilment on the author's part.