I can see that you’re financially savvy and trying to think about parents’ financial planning which is really nice. For a long horizon financial need, investment in S&P is good. But for a near term financial need (paying tuition in the next four years), return from S&P could be too uncertain - what if the market crashes by 40% in the next 3 years? Though, it sounds like you could still tap into your grandparents’ trust account in that scenario so maybe you’re still fine.