I think there's no harm in going through some LSAT prep literature and seeing how good you are at it. But, like most posters are saying, I don't think there's any advantage to taking the LSAT so early; there will be ample time to prepare in college, and, while I can only speak for myself, I definitely became much more intellectually mature over my four years of college.
If you think you have nothing to do, here are some ways to spend your summertime that I think would be much better than balls to the wall LSAT prep, off the top of my head:
1) Go to amazon, find some books that interest you, read them. It would be good to read some books related to your intended major/career path, but also to read some completely unrelated. Who knows, you might have a hidden desire to be a nuclear engineer.
2) Find a good exercise and nutrition plan, hit the gym on the reg (http://www.youtube.com/watch?v=wjvZOh4OzBs
), get in ridic shape, impress all the hotties come fall.
3) Buy a cheap guitar or keyboard, learn to play it, impress all the hotties come fall.
4) Enjoy yourself and others; go out and meet people, polish your charm, impress all the hotties come fall.