I’d be checking ID, asking why no child protection officer was present, or no parent, I’d want to check the head had been informed by their superiors and enquiring as to why he wasnt present
ID fair enough but I would imagine the uniform may be a giveaway!
Appropriate adult (whether a parent or someone from local social services etc) would be arranged for interview but not needed to be present during the arrest.
I guess the head depends on school policy or if they are that bothered ( I can imagine it happens more in some schools than others!).