The main problem you're going to have is the age factor; and people will either ignore the fact, not be bothered by it, or think you are far too young and can't do a professional job.
When I was fifteen, I was in the exact same position as you, and decided to set up a teaching studio for beginners to piano. I started with my first pupil when I was fifteen and built it up very gradually; now, seven years on I am absolutly fully booked, and have a waiting list for my pupils.
The point here, is that it takes time, determination and effort to set up a business that is sucessful in relative terms to your goal. I can't count how many thousands of fliers I (personally) delivered, how many adverts, phone calls and networking I did over those years - but it pays off! It really does!
So, I suggest this; start now, but don't expect to be making a lot of money this summer, it's doubtful it will happen. Having said that, next summer you could be earning a nice little wage, and the summer after this even more so. You should also consider how much you need your partner. Partnerships can be extremely irritiating in a business preliminary based on a solo venture, so make sure you're both hard workers and good friends so that you don't find that one of you is doing more than the other.