Can I strongly recommend 5% bonds, like with BND (Vanguard Total Bond ETF)?
It won't change a student's performance much, but it will introduce the concept of rebalancing. Each year, some effort will be needed to bring that BND investment back to 5%. Typically new investments are enough, but it might mean selling VIT to buy BND.
The key thing is when someone approaches retirement, if they're never rebalanced... how do they start? Decades of never holding bonds, and never selling VTI... it's not going to be easy. If, instead, they've been rebalancing into 5% bonds for decades, it's much easier to start changing the bond percentage upwards as retirement arrives. The 5% bonds is mostly for good behavior later.