Illustrator comes with most everything most designers need. There are really good specialized add-ons, like PD mentioned, that can be useful for specific purposes, but as you noticed, the good ones come with a hefty price tag.
In your situation (high school), you’re probably better off working on your design abilities than focusing too much on tech solutions to what are actually gray matter thinking problems. Way too many new graphic designers get sidetracked off into thinking that learning a bunch of programs will make them better designers, when, in reality, design isn’t so much about the tools as it is about designing effective solutions to visual problems using whatever tools might be needed.