I don't think there is anything like an easy promotion as you have to put in the groundwork to be considered regardless of anything else.
It's like the BS of over 18s needing to be 'of worth' before being allowed to stay in the ATC, when there is no similar requirement placed on adult staff. Because I'm pretty certain they know what would happen.
As Angus says, what are cadet NCOs for and how much time do they use whatever skills etc they might pick up and use to pass a course/board. Passing a course doesn't make you an NCO, it's the person inside the skin and no number of courses will make you a better / more suitable person. If as it seems the bar is too high, what is being attempted to be achieved? If a cadet doesn't pass where does that leave the unit commander needing the NCOs? If the cadet doesn't pass where is the incentive to stay around and try again, it's little consolation when you are teenager being told the standards are really high and there's no shame. It's only a hobby and with so much pressure elsewhere, you can get by without it.
We also need to consider what it looks like on a CV. Just by being an NCO can be a fillip and the people reading it won't know if you passed a course or not and are unlikely to care very much.
Then there is the new GCSE and A Levels which are moving away from the modular / 2 stage format, back to what I remember, ie study for 2 years (except GCSE is now 3 years starting in Yr9) and having to remember things to answer questions. As such how much time will some cadets have to do these additional things? The cadets we have in lower 6th have had more time off for homework (among other things) than previously as they haven't got a cushion anymore and have to know the stuff and schools and parents are ramping up the pressure.