WTF is going on with British primary school assessment?

I feel for my UK teacher friends... WTF is going on with British primary school assessment?
I always hated teaching English because languages, by their very nature, simply defy clear-cut, 'logical' rules and yet everywhere we're expected to teach rules. Teaching resources online always make bold, absolute statements about grammar points and I often had a niggling sense of doubt about them.
It turns out that linguists actually dispute the entire category of subordinating conjunction! I always found that hard to explain (or understand), apart from that it seems you can move subordinate clauses to the front of a sentence. But the oft-used definition of 'a subordinate clause is less important than the main clause' doesn't, to me, make sense, for example, of the coordinating conjunction 'so', which is basically the inverse of 'because'. As in, "I ate quickly because I was hungry." versus "I was hungry so I ate quickly." In either case, the reason for mentioning my hunger is in giving an explanation for the statement "I ate quickly".
Also 'but' - "I like comic book movies but I still haven't seen Batman v Superman." In that sentence, isn't the first clause subordinate to the second? Because you could replace 'but' in the middle with 'although' at the start, which is a subordinating conjunction?
I thought my own understanding of grammar, from my education and training leading up to me having to teach it, was just insufficient, but it seems like people who ought to know this stuff are equally disgruntled.
Example question from Y6 grammar test, taken from Michael Rosen's recent blog post:
Tick the sentence where the highlighted word is used as a subordinating conjunction.
Tick one.
He was at school BEFORE you.
She did her homework UNTIL dinnertime.
Do not undo your seatbelt, UNTIL the car has stopped.
WHEN the sun is out, we will go outside.
Just what in the hell? Can anyone answer this? It seems from Michael Rosen's update post that a lot of people who you'd think ought to be able to answer a Year 6 grammar question cannot.