Doris,
Next to your students list, use function =RAND() in an empty column. This
will generate random numbers between 0 and 1.
You can then sort by these numbers, so as to get a random student list, and
assign the first 20 to the first advisor, the next 20 to the second and so
on.
Note: you could use function =INT(RAND()*20)+1 instead to get random numbers
ranging from 1 to 20, which could be used directly as the advisor number,
but there's no guarantee that you'll get an even split (I tested it and got
numbers of students per advisor ranging between 5 and 15).
HTH,
Nikos