Let's start with two questions:
What is music theory?
Music theory is the science of harmony and melody.
Why is music theory important?
Music theory gives you the tools to explain and reproduce what going on musically for any melodic, harmonic, and rhythmic sound that you hear. Good music theory knowledge lets you pick any music apart and see what makes it work.
There are three things that make a great jazz musician. Language/Vocabulary, science, and creativity.
Language and vocabulary are developed by listening to recordings, going to concerts, and playing with people who are better than you.
Science is music theory and explains how it is all connected.
Creativity is art and is developed by always trying new things and staying objective abut your own playing.
When you first start playing jazz you struggle with the science because it's new. You might have a handle on the vocabulary if you listen to a lot of jazz (which you probably do because why else would you want to play it?) Creativity is developed by always trying to push your musical envelope and trying new things while not being afraid to make mistakes (kind of like life, eh?)
The science of music, that is music theory, gives you a head start when it comes to analyzing why Monty Alexander sounds great or what lick Charlie Parker played in Yard Bird Suite.
Why will summer music theory classes change your life?
The sooner you take music theory the sooner you will see what's going on behind the music you love.
You will hear a chord or a phrase and be able to explain it scientifically. True, jazz is about hearing and playing what you hear, but ears don't always come out of the box being able to do that and only a few of the cursed few have perfect pitch and/or photographic musical memory.
I say cursed because have you ever seen someone with perfect pitch try to play an out of tune piano?
When you hear a cool chord progression you can use music theory to quickly break it down to see how it is put together, and then you can recreate it at any time in any key because it's not just a sound anymore: it's a reproducible formula that you can use in any key in any musical situation at any time.
When you discover a new idea that you can break down using music theory, it makes it that much easier to practice which means you will internalize the idea more quickly and more efficiently so that is starts showing up in your playing without conscious thought.
Why study theory when jazz is all about your ears and playing what you hear?
Sure, jazz is all about having strong ears, but as I wrote above, most people have to strengthen their ears before they can play what they hear. As you study theory and ear training by analyzing and transcribing music, your ears will become stronger and at some point you won't really need to think about theory as you're playing, you will just hear something and play it, regardless of what key you're in, where you're starting, and what style/tempo the music is in.
This is like when you get time as a performer. Time usually "clicks" in people at some point and I've noticed that it often coincides with the moment when the big form of tunes start making sense. When the big form of tunes start making sense, that is, the form from beginning to end instead of A or B section, it gives the performer a chance to mentally relax and when a player isn't worried about what is coming next, that's when they finally relax which allows time to enter their playing. It sounds kind of strange, but that's how it often works especially for jazz players who are trained classically at first.
Studying college level music theory in high school puts you ahead of the curve and gives you easy A's.
My first two years of music theory at the University of Washington were a breeze. I coasted through and always had between a 3.5 and 4.0. Well, almost. There was a certain teacher, Mr. Beal, at the University of Washington who didn't like me. Possibly because I asked about inconsistencies in his lectures, or showed up late most of the time, or got perfect grades on tests while never turning in homework. Either way, he gave me a F-- (is that even possible?) last quarter of 2nd year music theory.
An F--. The crazy part is that it wasn't the first time because I also picked up an F-- in a poetry class I took at the University of Washington. I think that F-- was the result of my last poem getting personal with the poetry professor's ideology about poetry. She had something against rhyming and words that made sense when written next to each other and I thought that it was OK if a poem rhymed every once in a while.
My philosophy is either do something really well or do it super bad, but no matter what, don't get caught in the middle.
Back to the point of this post. If you want to do something interesting during the summer while you're in highschool or junior high, take a summer of music theory at your local community college or university. As a jazz musician you will not regret it because it simply puts you ahead of the curve, makes you smarter about music, and starts you early in the educational scene that is such an integral part of the real world music scene.
Hey, I'm too old for music theory classes.
No way man, you can take music theory at any time to get a boost in your playing and creativity. Just do it and be real: study, take the tests, make it matter.
If I'm not back in 24 hours, call the president.
Scot is available for skype jazz piano lessons (and google hangouts, phone call, etc...)
Use the contact link at the top of the page.