Have you read our article on making the transition yet? If you haven't, check it out here:

http://www.toesalad.com/articles/how-to-transition-to-minimal-running-shoes

I once heard someone say that barefoot running is about being strong, not fast. I think that it is possible to run fast minimally but the most important thing is to strengthen the feet where they have been lacking for so long. Once you get that down then you can work on speed. That may be difficult for an enthusiastic 13 year old running cross country! It takes patience. Good luck!