hsmomstef - thanks to Tokenadult (who has more than reciprocated with useful posts both here and on the aops forum) for stepping in. I've been aware of EPGY for years, but my son never actually used it. I provided the link because I like their guide to typical course sequences, not necessarily because I am recommending taking courses from them. My son self studied using materials we selected ourselves until after calc BC, then did distance learning thru
http://www.utexas.edu/cee/dec/ for multivariable and linear algebra (much cheaper than EPGY but minimal "instruction"). Then he started taking courses on campus at the local university. He has also done a lot of specialized math summer programs.
Zogoto - in most programs, linear algebra is where students are first introduced to writing serious mathematical proofs (not the kind you do in geometry). So it's a prereq for most upper division college math courses. If you have an interest in number theory, by all means consider taking it or studying it on your own. Some number theory courses assume you already have exposure to proof-writing at the level of linear algebra, but other don't.