Autofilling a numeric time code corresponding to a date?

Relative excel novice here, just learning proper vba script.

I have a work sheet with student scores across various competencies over time. I'm sorting the data
preparing ultimately to run a factor analyses / create a MTMM.

My sheet has 80,000 rows from all testing so the manual sort isn't going to work. I have the date at which each
student was tested but need to assign a number code (time 1, 2, 3 etc) for each student based on when their
first testing session was to their last. The issue is that when I autofill it does a series fill the whole way down or
just a straight copy. I need it to start back at 1 for each new student as it fills down the row i.e.:

Student    time
1                   1
1                   2
1                   3
2                   1
2                   2
2                   3
2                   4
n                  n

Problem being not all students were testing same number of times, there is lots of variance.

Any help is greatly appreciate, thank you very much.