You're giving a 15% growth rate with zero volatility? That isn't going to teach many important lessons.
How about offering a range of rates with volatility increasing as rate increases. Then they can think about the benefit of guaranteed return vs the benefit of long-term growth, or a combination of both.
How about offering a range of rates with volatility increasing as rate increases. Then they can think about the benefit of guaranteed return vs the benefit of long-term growth, or a combination of both.