Don't. Cold approaches are overrated. They don't work as well, and they aren't easy to pull off. Not impossible, but not easy without coming off as a creep.
The best approach is either to network or to hook up "by accident".
The first is simple. Networking is your biggest tool in highschool. Do you have friends? Do your friends have friends? Do your friends have female friends? Do these female friends have MORE female friends? Honestly, the more people you are introduced to through your friends, the better off you are. You'll build a large network of friends and meet some females along the way effortlessly, just make an effort to hang out in large groups and don't be one way about this. Introduce your new friends to other friends you have. with an army of friends, both male and female. Social proof - check. Female company - check. Now it's just a matter of picking out the girls you are interested in, and when you meet a new girl you might be interested in, game her from the start, don't wait til you are too familiar or it will be hard to avoid the friend card.
Second, is the accidental approach. If you overhear something you have a good one-liner for, say it. If you see a girl who looks like she might need help with something, offer it. If you run into a girl looking at something you are interested in, comment on it, strike up a conversation. It's fairly easy to do this as well.
The only thing that even comes close to a successful cold approach in high school is that first day thing. When you walk into a class for the first time, it's not unacceptable to sit down next to a cute girl and introduce yourself and strike up a convo. That's also a clean way to meet girls.