1. Spend a reasonable amount of time in your school library.
This helps you look smart in a couple of different ways. It also doesn't hurt if you check out books frequently. Not only with your classmates see you and think "wow what a nerd" (always the goal), your teachers will see you. And teachers talk. This helps in creating a good reputation for the rest of your high school career. I'd say the same for college but I'm still a wee lad and I doubt professors care enough or you'd have the time.
2. Nod.
What? What does nodding have to do with anything? Well, it makes it look like you understand what's going on. If you're unfortunate enough to get caught in a conversation with people who are actually intelligent, nodding will make you fit in. Pick a person who seems the smartest and just agree with them. Don't overdo it because then they'll assume you know enough to talk.
3. Look inquisitive.
This will make people around you assume you are thinking about something confusing that requires concentration. In reality, you could be thinking about if you closed the garage. One thing that helps you look inquisitive and fascinated my something, is to try to reimagine a certain recent event as vividly as possible. You will find that while you do this, your eyes wander and it looks like you're thinking very very hard.
4. Have quirky mannerisms.
Thank Hollywood and TV for this one. All the famous geniuses in movies are very quirky. From their expressions to their posture.
5. Speak quickly.
Not always, but sometimes speak incredibly fast so the person has to ask you to repeat. Sigh and repeat it slowly in the most respectfully condescending way possible. Even if you're explaining how to add numbers sound like a know-it-all. Of course, you can't do this too often or you'll just seem like a word I can't publish here.
