Primary school teachers tend to be nice people, who aren't terribly keen on confrontation, have a track record of perhaps being insufficiently sceptical (Brain Gym, and longer ago ITA) and have a generally optimistic and trusting view of others.
Christian extremists tend to be cynical, skilled manipulators and liars, whose pronouncements on things like young-earth creationism can appear dangerously plausible if you aren't aware that YEC even exists as thing.
It's not a fair fight.
Anyone aware of how easily Trot entryists took over constituency Labour parties in the 1980s will sympathise with the school. Once there's a critical mass of people who turn up early, stay late, sit on unpopular committees and generally provide the sweat and hard work, it's very difficult to say "hang on, these people aren't our people". Eight people have apparently been excluded from the school in question, so the take-over was clearly well resourced, well planned and well executed.
You can mock the naiveté of the (one half of a job share) head, but I suspect that primary heads don't have extremist entryism high up their list of concerns. It would be interesting to know how so many non-EU citizens obtained whatever the Scottish equivalent of CRB checks is, and quite how many of them had work permits, but I suspect that their mostly being volunteers is a large part of the problem; volunteers may or may not be properly CRB'd (my experience is that schools vary on this), but if they're not being paid the governance associated with "employing" them will certainly be weaker than if they're on the pay roll.
The letter she sent out is, of course, career-endingly stupid. Primary schools are not the place for "balanced coverage", "teaching the controversy" and all the rest of the YEC "wedge issue" nonsense, even if you accept (as obviously I don't) that such books are balance or there is a real controversy. The whole idea that schools should distribute books they are given without analysing them, and that somehow they should be given a free pass because they are free, is absolutely wrong. But the whole saga of Kitzmuller v Dover Board of Education shows you how insidious these people can be, and how willing they are to lie and deceive in order to get their "ideas" over.
The head will presumably be sacked, and if she isn't I should imagine that the governors and/or the LEA will be having a firm meeting with her. Coffee and biscuits may well not be offered. It's possible that she was party to the scam, but it's unlikely: I think she's guilty of ignorance and naiveté about something which is not really within her remit. Sad.