His Dirty Little Secret by jamesgirl
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=24000&warning=NC-17Summary: William 'Spike' Giles and Buffy Summers are both students at Sunnydale High, but they lead two very different lives. Spike has it all – looks, wealth and popularity. Buffy has nothing – since her parents divorced, she's been living in the servants' quarters of the Giles mansion, where her mother earns a meager living as a maid. But beyond these differences, they have a secret relationship – at school, Spike pretends not to know her, but at night he visits her room with growing regularity...