Oops. Sorry about that.
<<SUM Sales_by_School.Nbr_CMs_sc@'#,###'>> returned the correct value when the query is un-filtered, but not when it's filtered.
The 2nd one didn't return a value.
So, I resorted to adding School.Nbr_CMs_CALC and added a rule to calc that field instead pulling it in via shortcut.
I guess the fact that the shortcut has two .'s in it causes the issue with the aggregate function.