I agree that the Roald Dahl translation is both forced and unnecessary. However, I think that it should be taken up with the teacher as inappropriate rather than knee-jerkingly blame the SNP. And having it as the "reader" seems particularly stupid.
I came across this briefing paper when checking to see if the Scots language was a compulsory part of the CfE. While I haven't checked all of the links in it (which give further background), it suggests that Scots could be introduced as the 2nd MFL in primary school (ie in the upper years, after the 1st MFL, which is started in P1), should teachers want to (no obligation to do so) and explains some of the ways it could enrich a child's learning experience. No hint of being forced to use the language.
I did find the briefing paper interesting in explaining why Scots should be considered a language - but there again, I'm a linguist 
FWIW - I had to "learn" a form of Scots to read Sunset Song, back in 1977/78, for my Higher English - loooooong before the rise of the SNP to any form of power. This was in the same nice middle class area that Luna lives in, so there was no political motivation or local speakers other than the fact that it was good literature.
And someone else has said that The Cone Gatherers was in the Higher Curriculum (I know it's not written in Scots but it's part of the Scottish texts curriculum) back in 1989 - so again, not something that can be blamed on the SNP.
FWIW - I a posh Glaswegian brought up in Bearsden/Milngavie know what an oxter is 