Just because only a small percentage of high school relationships last doesn't mean that yours won't. However, it is important to not get so caught up in thinking about what might or might not happen in the future that you forget about living in the moment.
College is still a long ways off and you have several years left until you need to decide what to do. In the meantime, why not focus on today? Be grateful for your time together now and cherish each moment that you have with each other. Focus on building a strong, supportive relationship. Most importantly, though, have fun and enjoy each other's company.
As strange as it may sound, living in the moment and enjoying the little things is the best way to ensure that you have a future together. After all, the more time you spend loving, cherishing and appreciating each other, the stronger your relationship will become. Then, when the time does eventually come to decide what to do about moving to another state or going to college, your relationship will be strong enough to overcome whatever challenges you may face.
Even if you don't make it past high school, you never know what will happen in the future. Check out these high school sweethearts who reunited 62 years after they graduated :)