I've attached a packaged workbook to demonstrate a possible solution. I'm using the steps detailed for the Table Calculation option for customizing grand totals as described here: http://drawingwithnumbers.artisart.org/customizing-grand-totals-part-2/
Note that the solution involves duplicating the School Code field. This allows you to address that field in the table calc, which is essential because when in the Grand Total the original School Code field will be "All" (all values), so there is no way to get the averages for each code (the window size is 1). But using the value when First() = 0 allows you to only perform the calculation once in the grand total. I've not included the additional windowing optimization as the Tableau 8 data engine will not have the same issues present in 7.
Also, note that Tableau 8 does stack all marks by default, so you might notice some difference if you turn it off and on from the Analysis --> Stack Marks menu options.
Custom Grand Total.twbx.zip 18.1 KB
Thank you very much Joshua. I was assuming that simply dragging and dropping the dimension field in the level of detail shelf I was creating a copy, bu I had to duplicate the dimension first and then include the copy.
Thanks for the help!