Most Helpful Guy
If you were a college student this would be one thing.
NOBODY WILL THINK A 15 YEAR OLD GOT A BOOB JOB. In not sure there's a doctor in the country who would even be willing to perform one.
Most Helpful Girl
Perhaps try gradually (like over the course of a few months) loosening the binder little-by-little, and when the binder is loose enough, you can work your way to sports bras, and then eventually to just a regular bra; that way, the people who are used to seeing you daily (like at school) will not be so shocked when you finally get to the point of wearing just a regular bra.