My organisation has public liability insurance and we often host other organisations doing their own thing. Our insurers told us that as long as the activity was within the usual business of our organisation, and invited in and sanctioned by our organisation, our insurance would cover the public attending.
Has she asked the school if they could check with their insurers (probably via the LEA) whether she might actually be covered by their policy? or whether it could be extended to cover her activity for a small additional cost?