But having a child that went to the school it's relevant to mention how well the woman normally did her job and how good she was at it.
I can see it from the LEA's perspective, though. A well-organised and motivated group of entryists convinced her of their bona fides to the point that they were operating all across the school, and were able to distribute literature without any checks being carried it. As it happens, they were (generally) harmless nutters: it's unlikely that beyond angering some parents and causing some tense conversations in some houses much long-, or even short-term harm was done. Stupid, offensive and lacking in care and insight, but ultimately harmless.
However, as tabby alludes, this happened "around the side" of normal process. How many schools have people working in classrooms who are neither (a) parents/grandparents nor (b) employees who have been through a complete application, interview and references process? Even if these entryists had CRB checks (which, for foreign citizens, are close to meaningless), why did no-one ask "why are these people volunteering at this school?"
Parents and grandparents have an obvious motivation and link to the school; employees are interviewed and "why do you want this job?" is a pretty straightforward question, and being paid is a pretty clear motive. Why were these people (eight of them, it would appear) offering to work, for free, at a primary school? You don't have to be paranoid who sees abusers behind every tree to think that a headmistress sufficiently gullible to allow that to happen wouldn't have noticed had they been, say, inviting children back to meetings in their houses which it's important that they don't tell their parents about because they won't understand. It's a safeguarding nightmare. If you turned up at a school, with no connection to it, and said "I want to volunteer in the classrooms, no need to pay me, just being with the children is reward enough", a sane school will want chapter and verse on why you want to do this. That, it would appear, broke down. And it's hard to say "how good she was at it" with a safeguarding disaster like this. CRB checks aren't enough: what also matters is why people are in the classroom.