I would think that the majority of trainees get through, you shouldn't get through the application if you can't pass the course but there are some who don't make it for whatever reason, not always that they 'fail' e course but often that they decide they don't like the job and that it isn't for them.
There are exams along the way but the trainers will do everything they can to help you pass, they don't want you to fail. There arnt so much pass or fail marks for these, you answer them, if you get them wrong you review them (normally the trainer re-words the question or puts it into a scenario to check if you do know what you need to know). I doubt you would do that badly in one of the exams as your trainer would know there are problems long before you sit any tests.
The practice side of it is just down to confidence. This comes with practice and your minder driver will help you with that. The 220 hours or whatever trainees need to do now a days is a minimum and neither your minder or manager will bother putting you forward for a competence assessment until they both feel you are ready and until you agree that you are ready.
Whilst there is homework and extra reading needed during the training it's not a case of sitting up reading through books and making notes 24/7 for a year-pay attention in the training, ask questions if you don't understand stuff and just keep reading through notes every so often to keep your knowledge up. You shouldn't need to sit up all night before any exams cramming revision in.
