As a junior in high school, I would take the time to study math at your leisure. While a systematic treatment provided in a textbook may be useful if you're looking for college credit, you may wish to consider studying math in other ways to build your appreciation of its many facets. There are popular articles and books written by Martin Gardner and others on mathematics.

In addition, the American Mathematical Society (AMS) publishes a number of books for the Student Mathematical Library that are short (100-150 small pages) and focused but allow you to explore an interesting aspect of mathematics with (often) nothing more than an understanding of a year's calculus. You may also want to visit a local university library and read the periodicals published by the Mathematical Association of America (MAA) geared towards undergraduates such as the Mathematics Magazine or the College Mathematics Journal. (You can have them delivered to your door if you join the MAA as a member (around $25-$35 for undergraduates).

If you're heart is set on moving forward rigorously, why not consider taking a course at a local university or college and receiving college credit for your studies? I would NEVER discourage self-study, but if you wish to become a mathematician, you will need to go through some hoops. A bachelors degree with a major in mathematics is probably one of them. Why not advance that goal now or over the summer? Also, you might consider linear algebra!

Good luck with your studies and don't neglect your social life!

Dov