I really want to join my school's jazz band on guitar, but it's a very select group and not easy to get into.
I talked to the teacher today about it and she said I need to read music, which I can do semi-fluently, as in I can read notes but in terms of figuring the best way to play fingerins but if there's a note in front of me, I can play it, and rhythm-wise I'm fine. The next thing was chord changes, which I have a vague idea of but if someone could explain exactly what that is in jazz, it would be great. The last thing she said was improv, which I have no idea how to do in a jazz context. So, if anyone could point me in the right direction, that would be great.
sounds like she may want you to have a tad more experience. If you're not graduating anytime soon, I'd tighten up your jazz skills and you can join next year?

if you have no clue how to improv to jazz you should listen to some good jazz (of which I have no examples of off the top of my head)

and learn as many scales as you can

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