[R] function to collapse data by factor/group

[Alexander Nervedi]

Hi

is there an R equivalent of the Stata collpase command?

suppose i have:

classes <- 1:5
schools <- letters[1:5]
stud <- 1:10

dat <- expand.grid(School = schools, Grade = classes, Student.ID = stud)
with(dat, table(Student.ID, School))
dat$marks <- rnorm(nrow(dat), 50, 25)


I  want to get the mean score by school and class  while the data is at the 
school-class-student level. I could write something up but I was wondering 
if there is already something otu there.

Alex