That's not how it works. If you had 100% immunity in the young, the old would still be at relatively similar risk because of homophily in social networks. That is, old people spend most of their time with old people. Herd immunity for the general population requires a homogenous mixture of social interactions across those infected.

Keeping the vulnerable protected sounds good and all, but in the case of, say, children, the vulnerable is the teaching staff.