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Hi,
Using the traditional Pivot Table this cannot be done. Since you are using Excel 2010, you can do this by downloading installing the free PowerPivot add-in from Microsoft.
Once downloaded and installed, you may write a simple calculated field formula in the Pivot Table as
=DISTINCTCOUNT([Class Date])
If you do not want to install the PowerPivot add-in, then you may use a formula based solution as shown at this link on my website
http://www.ashishmathur.com/count-uniques-with-conditions/
Hope this helps.