Don't know what the failure rate is (have heard about 70% for initials), but all 4 people in my class recommended for the CFI practical all passed on their first try (myself included). That probably also has to do with the quality instruction we received, but I also believe it was because we all hit the books. Honestly, practice teaching as much as you can before getting your CFI (in the presence of another CFI of course) to help develop your teaching skills.

Good luck!