Tell me a story about him/her (normally a him though, isn't it). You know the type - dressed in army fatigues, general sense of weirdness, quiet but with unexpected outbursts, air of a serial killer.
Ours was called Sebastian. He'd been to every primary school in town before being kicked out of each one, so when we got to secondary school, everyone knew him but few liked him. He once made a 'cherry bomb' out of batteries which he left on our French teacher's chair. It clearly didn't work as intended.
The oddest thing about him (or maybe what made him so weird) is that his dad was the head of our 6th form college. While I was there, we went on a college trip to Russia. Because I was a renowned swot/helpful type, the head pulled me to one side before the trip set off, and begged me to 'keep an eye on Sebastian'. I failed miserably at this task, partly because I couldn't bring myself to talk to him much, but mostly because he was a psychopath. He smuggled a ball of hash in cling film in his mouth through Russian customs and showed it to us proudly afterwards. Then he got paralytic on vodka before we went on a long train journey and had to be bundled on by the teachers.
My guess is that now he's CEO of some kind of city bank. Probably while secretly murdering people though.