You are just a kid now, concentrate on studies. For business you need experience. Blah blah blah… People will have numerous reasons for why you shouldn't start a business, all out of good intentions only.
But before you gulp down all that, think for a while about Bill Gates who left Harvard to start Microsoft; Michael Dell who left the University of Texas to start Dell; Milton Hershey who opened his first candy shop when he was 18; Fred Smith who, while attending Yale, received a "C" on his Fedex business plan and decided to start his business anyway; Steve Jobs who left Reed University to start Apple; William Hewlett and David Packard who started HP out of a garage after graduating from Stanford, or the thousands of other young people who have started a business and been successful.
What would have happened if they were convinced by these lame arguments and backed away from their business plan?? We would have had to live without a Dell, a Microsoft, an HP, a Hersheys, a Fedex or an Apple… oh! Horror horror!!!
Some ways to tip the card in your favor:
Adults expect less from young people and it can be used to your advantage. It's OK if you're not perfectly polished. It will take less effort to please clients and make a name for yourself with the media.
There's little competition from other students, which makes your story more press-, scholarship-, competition-, client- and award-worthy.
There are many nonprofit organizations and individuals that support youthful endeavors.
First on this list is your school, which probably has teachers who have contacts in the business community that can help you.
Students often have income coming in from their parents. Even if it's not consistent, then it's at least something you know you can always look for. If you're venture fails when you're young, you definitely won't starve or lose your house. The practical knowledge you gain from running your business can help your academic work and vice-versa. Some schools will allow you to earn academic credit from an independent study of your business. You can also base class projects on your business.
Young people have a fresh perspective on the world. This perspective helps them to see many opportunities that till now have not been exploited. The founders of Microsoft, Dell, HP, Hersheys, Apple and Forex will vouch for you.