How can i grow the $15k in my savings more at 17?

First, you should do well in your studies + aim for scholarships + bursaries!

• work part time job while you study if you can cope
• put $ into Robo-Advisors/ high interest savings account

I think if given such circumstances. While investing maybe a third of the 15k into a broad index fund. Actually the runway you have is really decent. So while buying into the index i would personally try to look around for a single or maybe 2 stocks with high conviction and invest the rest in those. Since u already have some bonds and can put some in an index. But of course this is up to your own risk appetite and when you are looking to reaccess this pot of gold. But i think there are many great opportunities now in the current market and alot of good can come off learning how to pick stocks.
To me growing the initial wealth requires a slightly more concentrated portfolio. But this is just my view