Don't go and try to change yourself. You'll look back on it and realize it was a mistake, trust me, I've tried to go punk and it really didn't work because I wasn't being who I was.
But if you really want to be mysterious, then try to stay in the background and don't really pay others' attention. Don't really talk that much, not never talk but don't talk about yourself. Don't scowl at people and don't dye your hair, it makes you look like a wannabe. A poker face would be fine. And don't smirk because other people will think your laughing at them and avoid you. Don't wear anything that will make you stand out very much, not neccesarily dark colours, and nothing revealing. While your at school don't ignore people, let them know your at least listening. Participate in class less, but don't overdo it. If they don't know anything about you, then you've made yourself mysterious. But sometimes that may not be a good thing.