If you can be a bit flexible with the position of the percentages, try adding the field being counted (number of students) into the Values area of the pivot table. Field settings for the second instance of the field will be "sum" and Show Values As will be "% or Grand Total".
Good luck with it.

Thanks for your response. For some reason, the subtotals do not calculate properly. I am trying to get percentages of a particular column value. That means the numerator and denominator are in the same column of data. The "Show values as" functionality calculates the percentages for each row total even when I select "% Of" and specify the field. The dialog box wants me to specify a base, which I don't understand.

So, for example, here is what columns look like:
Jan Feb
Signed up 10 12
Attended 8 9
Cancelled 1 1
Missed 1 2

What I want to see are the following ratios:

% attended 80% 75%
% cancelled 10% 8%
% missed 10% 17%

For some reason, the formula returns the percentage of the row:

% attended 47% 53%
% cancelled 50% 50%
% missed 33% 67%