You are right in some of what you say tbf. But there are changes afoot.
So basically the curriculum as it stood/stands allowed certain stuff to be taught to primary but there are motions to apply sex education into it by 2020.
Under older legislation you are right it was secondary when certain elements came into it. So puberty before puberty etc.
In this schools case the deputy head designed something specifically and started to implement it under the relationships agenda, which primary's have, but the content was not what falls under relationships, but under sex education. If that makes sense
I can understand your concern about about 2020 but trust me on this, when it comes to new initiative changes in the curriculum they change like the wind at any given time
The number of curriculum proposals I've seen being scrapped overnight is ridiculous - and it's nearly always been for political reasons. The biggest two I can think of were the 2010 New Primary Curriculum scrapped within days of the GE, and then the 2014 curriculum that was brought in. Both decisions based on the government at the time and what floats the boat of the Education Secretary at the time.

