To promote yourself you can start to offer designing free of charge, this will add to your portfolio and line of connections, that if your work is good or become better in time then work on larger projects and demands. If you already think you are good enough and have previous work to impress potential clients then advertise your services here and on other similar forums.

There are always clients looking for something and it may be your work. At early stages in to freelancing there is rejection, but if it is your passion keep pursuing it. The more experience, quality work and delivery the better.

Keep your job for now or find the one that you can be flexible with for your classes. I'm in a similar situation to you, I'm 16, and doing freelance work with school (I also have four classes) which is a bit hard. Just organizing myself was all the help I needed and experience comes to naturally.

My advice may not be the best, but there's many ways to approach things, and I'm also still learning.