I think it's a question of gently balancing different needs. Within a multi-racial school in particular, it might be a bit too 'full on' to do the blacking-up without some minor allowances for possible misunderstanding, and the real resultant dangers of alienation, especially if having to explain intricate historical details for origins of tradition - which the folk community themselves seem unable to agree upon.
Imagine a young black child going home and telling their African or Asian mum and dad that they saw funny bearded white men dancing around with black painted faces at school. Yes, education needs to be firmly in place, but we don't even have it in place within those English village schools, *without* a real black face to be seen. Most English white kids wouldn't understand it, so how can we expect immigrant families to do so?
Hypothesising for a moment that the blacking practice is indeed *merely* a form of guising, then adapting to *context* is thoroughly appropriate to the maintenace of the tradition. For if, as Borchester Echo says, it was about use of those materials which were readily available - then the particular *colour* was purely *incidental* to available materials, and not an essential fixture of any region.
I'd argue that flexibilty be allowed for. Certainly in this instance, the Morris Men could have suggested to the school that they could mildly adapt their masking/guising for this particularly racially sensitive context.