JSngry (and others) and Jim Alfredson are right, Selmer and Keilwerth are two of the most highly regarded saxophone names. I played both of those for many years, but last year I tried some Cannonball horns, really liked 'em, and that's what I've been playing since. They're good, solid horns that play great, and they're not as wildly over-priced as lots of other horns.
However, before you rush out to buy a new horn, realize that you can probably hang for awhile just by making sure that your Jupiter is in good adjustment (lots of moving parts on a sax), making sure that your mouthpiece is straight (level table, even side-rails, good tip), and having good reeds that seal and respond the way you want. Cover all those variables, and you'll probably be way ahead (equipment wise) of most other players in your school band. Plus, you can take the money you would have spent on the new horn and buy a bunch of CDs and hook up with a good teacher in your area.
Apparently you're in Arizona, so you might want to contact Tim McAllister. He's the (classical) saxophone teacher at U of AZ, and he is an incredible world-class player and teacher. He'll probably have some good suggestions for sax teachers (jazz or classical) in your area, and he would be a great resource for advice (besides all the friendly folks here).