Why does a boxplot in ggplot requires axis x and y?

There are no fancy statistics happening here. boxplot is simply assuming that since you've given it a single vector, that you want a single box in your boxplot. ggplot and geom_histogram simply don't make that assumption.

If you want a bit less typing, you can do this:

qplot(y=escuelas$ceroonce, x= 1, geom = "boxplot")

ggplot2 will automatically create a vector of 1s equal in length to the length of escuelas$ceroonce

This could work for you:

ggplot(escuelas, aes(x= "", y=ceroncee)) + geom_boxplot()

ggplot(escuelas, aes(x="ceroonce", y=ceroonce))+geom_boxplot()

ggplot will interpret the character string "ceroonce" as a vector with the same length as the ceroonce column and it will give the result you're looking for.