So the SubForm query would need a default value in the criteria line.
You would want to use the Forms!Formname notation.  See Value From a Form

So you need to have =Forms!ClientF!ClassEmployeeF.Form!ClassTypeId

Check your Link Child Field and Link Master Field properties in the subform. If that's the key field linking the two together, it should be automatically set.

Scott: I think you're going backwards. The subform should be getting that value from the parent.

And are you sure this needs to be a many-to-many relationship? If you're enrolling students in a specific class, it's just a one-to-many.

If the ClassID relates to ClassTypeID, then that is also a one-to-many. So you've actually got a double one-to-many relationship here.

ClassTypeID > ClassID > StudentID

In this case technically the ClassT table would be a junction table bridging ClassType and Student. LOL.

You may be better off using nested subforms here.

ClassType ------------------------
     Class ----------------------
          Student
          Student
     -----------------------------
--------------------------------------

Richard you're correct.  I flip flopped the pictures.  Then I don't think I even type the names correct!
Evidently the coffee hadn't kicked in yet.

Julia - Sorry!  Richard does bring up some good points above as well as my oops.