The Club where I used to be CFI had a very straightforward solution:
1. You can sit any exams at any time. Just book a time. And they're free for a first attempt, but will attract a fee for the second attempt. Fail twice and we won't let you try again until you've been to a specialist ground instructor.
2. No solo circuit consolidation nor any further dual flights until you've passed Air Law, HP&L and Communications(PPL).
3. No solo navigation nor any further dual flights until you've passed Navigation, FP&P and Meteorology.
4. No Q X-C until you've passed RTF practical.
5. BAD WEATHER FOR FLYING = GOOD WEATHER FOR EXAMS!