Starting at 15 still isn't too old for you to achieve dancing at Championship level. Both posters here need to realize that I don't mean you will win a major Championship, but rather that, even starting at age 15, you will have enough time in your career to someday be able to enter Championship-level events and not look out of place. I can't make predictions more certain than that. But what I am saying is that your goal is not crazy at all. Furthermore, it's great that you're getting started NOW, as a teenager, rather than a few years later.
Now for advice: my biggest piece of advice for both girls is to find an Amateur partner as soon as possible. This doesn't have to be the partner that you stick with all the way to Championship level...it just needs to be someone to start working with so you can be heading down the path toward your goal. If you're at Fred Astaire, chances are very high that you'll be encouraged to do Pro/Am and not Amateur events. While Pro/Am can be a good learning tool and a quick way to get into competitions, to reach Championship level you MUST develop the kind of skills that are required in peer-to-peer Amateur partnerships.
The good news is that if you really are as talented as your teachers say you are, then they should nurture you by helping you to find suitable boys to dance with rather than just keeping you in Pro/Am. They should also help you by pointing you toward the best coaches you can get, even if that means giving you away as a student.
Unfortunately, not all teachers are this altruistic. If you find that yours doesn't want you to find an Amateur partner and doesn't support you going to the best coaches in town, then you should also find a more understanding teacher who truly wants to help you develop as a DanceSport competitor.
Good luck to you both, this is an exciting path you've set yourselves on!