I was an instructor there for almost 4 years (left last year) and was a TC before I left. Most of the attrition is due to poor performance on exams, there isn't much we can do about that, or from students that "can't take it" and choose the medical route.
As far as exams go, the 50% exam is only to determine your performance and see how well you'll do on the Comp. If you fail the 50 twice, nothing happens. Infact, no one even looks at if if you pass, so essentially I could take your exam, plug random numbers into the computer to ensure you pass and shred the exam with out ever looking at it. However, we would rather grade it to see the ridiculous answers students provide (everyone loves a good laugh). The Comp is different, if you fail twice, you're out. That might sound like a filter, but we tell you exactly how to answer the questions ("shotgunning" is allowed and pretty much required if you want to do well), and most good End of Card checkouts should have common Comp questions in them, mine always did. Plus there have even been a few who got to take the test a third time. Students have failed so many watches they have to get another page added to their books (the most I saw was over 20 watches and he still failed after NR declined his third Final Watch Board). When we get you ready for your final oral board we pretty much tell you exactly what is going to be asked, which is obvious if you have paid attention the the types of questions up to that point. If I wanted someone to pass I knew what to ask and what to stay away from. It pretty much like a script. I could still tell you what they will ask during your oral board (as long as you are an MM) The only student I failed at an oral board was basically doing the exact opposite Immediate Actions, twice (killed everyone the first time and tried to on the second casualty), hard to pull a passing grade from that. There are plenty of students that I thought would be terrible as an operator, had poor grades, poor performance, and a bad attitude that still made it. Infact, my entire crew thought one should fail, we told his board members to fail him , and they still passed him. (This is usually due to the civilians, they always pass you the second time). A friend of mine is on a sub with one of our ex-students who passed but shouldn't have, and the kid is still terrible, can't do a thing right and sucks at life in general. That being said, prototype has to be a pump, it wouldn't work any other way. Some people don't even see a need for it. It wasn't designed to make you the worlds best operator. When you show up to your first Ship (or boat for the tubers) all they expect you to do is talk on phones, take a set of logs, know how to qualify and (oddly enough) live on your own. It's funny but one of prototypes teachings is how to live in town on your own. Most sailors are out of high school and have no idea how to, which is why we are so into you personal life and it's one of the first places we look when things start to go bad.