Crow Sister has a good point, and I'd never really thought of it that way before. Morris traditions are not set in stone, and the disguise could be tweaked a bit by the use other colours, or by drawing black patterns on the face rather than going for that all-over blackness which could lead to confusion about its purpose.
And local schools, in their turn, have a duty to educate their children and the wider community that they claim to be supporting, by encouraging the morris to explain (or getting the teachers to explain) what the blacking-up is all about, and its place in our social history.
As to why the teachers all seem to be female and white - well, the great majority of primary school teachers are female and white in England. I don't expect Gravesend is any different to most places - I'd be surprised if it had a large, settled non-white community, and primary teaching is simply one of those careers (like nursing) that attracts far more women than men, almost everywhere. There's nothing very sinister about it.