Even though 4-year bachelor's degrees are more common, in my experience, graphic design is definitely an area where you can get a degree from a 2-year community college and still thrive! There's a lot of opportunity for freelance work and running your own small business, so I don't think your dream is unobtainable at all!
Considering you're a junior in high school and only have one course under your belt, I'd say the best next step is to sign up for more foundational courses in your senior year to learn how to use industry standard design software (Adobe creative Cloud, especially Photoshop, InDesign, and Illustrator), and also some art courses to learn some fundamental theories of design and composition, like color theory.
For now, I would definitely suggest that you continue to build your portfolio through your courses. Good supplementary courses are Udemy courses and EdX or other MOOCs (like this one). Once you have some more foundational design knowledge, then you could start working on prompts on your own. If you google "graphic design prompts," this is the first result on google. There's also tons of logo prompts out there, it just takes a simple google search to find them.
Next, probably in your second year at college, you could start doing actual design work for others, like entering into contests, doing a 99 Designs logo, or finding volunteer work on Catchafire. That will start bringing semi-professional work into your student portfolio. This will put you in a good position when you graduate to set yourself up for a job in a design firm or to use your portfolio to attract customers if you want to go freelance.
Good luck!