I did the ground school for air law and I found it a bit of a waste of time, the very large school in question just put the Pratt book on slides and read it out without adding much. I used it as more of an incentive to get started on the exams and then I self learnt the rest. This is what I think about the exams
Air Law - Very (very) dry subject because there isn't a great deal of understanding, just learning but it is ok to do yourself if you manage to keep your eyes open. Sitting on a wooden chair is advisable
Navigation - Shouldn't be a problem to self learn once you have started cross country.
Meterology - Some tricky concepts, worth the groundschool or asking an instructor to clarify bits and pieces
Human Factors - Mainly factual but relatively interesting, definetly self-learnable
R/T - "
Technical - Fair bit to learn but you can do most of it yourself
Performance and planning - Always better once you have done some practical performance and planning with your instructor
Hope this helps