I think the point being missed by the Head Teachers (and others) is that the legislation is aimed at Health and Safety AT WORK!!!! and does not address the needs of Students in school except indirectly. Even when calculating First Aid provision the numbers of Students is not taken into account in the legislation.
As a teacher supervising students who are playing conkers you have a Right to protective clothing (gloves, eye protection etc.) as appropriate, provided by your employers, the students are not altho' the activity of playing conkers should have a proper Risk Assessment caried out which would examine any hazards that might arise in the activity and set out any precautions and safety provisions that might need to be met if it is happening on school premises.