Financial Edge Investment Banking Course

Was a SA at a top BB last year, training was provided by FE and it was pretty good. They explained accounting and modelling in detail so you dont have to have too much prior knowledge although a high school kid may still find it somewhat challenging at times.

They also provide you with lots of training materials and walk throughs so you can always practice.

It sounds like you're miles ahead, so good for you. I wouldn't worry about any kind of in-depth accounting or valuation training because you will forget it by the time you are applying for SA positions anyways, and it may be difficult to grasp as a high schooler. Understand the basics, but then spend your time developing a solid understanding of the markets. You can also try out some stock pitch competitions. I think there are a few available to high schoolers. Personally, if there is any skill that is worth developing at a younger age, it's being able to communicate effectively and persuade others.