probability combination ?

This is a partitioning problem. To the 10 students, add 4 markers to separate As from Bs, Bs from Cs, Cs from Ds, and Ds from Fs.

Now you have 14 items (students and markers) that can be placed in a row. The As will be all of the students to the left of the first (leftmost) marker; the Bs will be all of the students between the first and second markers, etc. If two markers are adjacent, then there are no students who received the grade that is represented by the space between those markers.

There are C(14,4) ways to position the markers. These positions define all of the possible histograms.

C(n,r) is the number of combinations of n things taken r at a time.