nested sampling of a data.frame in R

In data.frame p below, there are 757 unique district names (dname) & 5210 unique school names (sname).

I was wondering how to sample 126 snames (schools) from 40 dnames (districts) in R?

So, in the final sample (say X), dim(table(X$dname, X$sname)) must return: > [1] 40 126

In a sense, this is multi-stage sampling, so I'm open to any packages.

p <- read.csv("https://raw.githubusercontent.com/hkil/m/master/a.csv")