DX (probably what you have) is by far the best plastic for beginners. The champion plastic is accualy the shortest plastic. However, champion plastic makes discs more stable, or overstable, meaning they cover tecnique flaws.
Keep throwing what you have, cheetas are alright beginner drivers, so are leapords.
If they are bugging you to get champ plastic, remind them that a DX Valkarye is the world record distance disc. and that (blake not sure about this one) but champ and z plastics have only broken the 600' barrier a couple times?
Mark, the best advice you will find will be on this forum, the articles on discgolfreview, and your local pros. The kids at your high school may be able to throw further than you, but that dosnt mean they know the best way or discs for you to throw. Keep up with stuff here and love the game, and you will probably be surpassing them in short time.
Z Pred-ESP Cyclone-Z Force-Z Aftershock-Z Comet-Ion-Pro Rhyno