Yikes, you're only 15? Thats a hefty piece of cash for someone your age. To answer your question, yeah you could grow anything with that light over a 12g. Remember, you'll run into one issue with a bulb this powerful over a tank that small... Heat. Being as young and cash conscious as you are, you may benefit more from starting with one of these and seeing how things go. If you have room for a second and heat issues don't creep up on you, you could consider a second of those kits.
What corals are you planning on growing (sorry, should have asked that first)
What corals are you planning on growing (sorry, should have asked that first)