Report Running Sum for sub Groups.

I have a report that is broken up into 3 groups.

District

   Student

      Service

I am trying to get a running sum for each of them. 

If I select a textbox  with runing sum over group it continues to sum past the group.

Hopefully the folowing will explain what Im trying to do.

District a >>> total Both studentt a and b

   Studenta >>> total services a and b

      Service a

      Service b

   StudentB >>> total services a and b

      Service a

      Service b

      Service c

District b >>> total Both studentt c and d

   Studentc >>> total services a

      Service a

   Studentd >>> total services a , b and c

      Service a

      Service b

      Service c

Currently it will total up all students regardless of what sub group there in. Is there a way to have it clear its self after each group so it starts over?

Thanks for any help.