I doubt there are any members of ethnic groups support his performances by attending.

Well they couldn't if they wanted, since he died in 2007.

So far as blackface and offence is conbcerned, I would suspect that it's the idea and not the reality that's the problem. Say "black make up" and the image people have is of performers dressed up and behaving in a way that parodies black people. Which in this case could well be the image likely to be conveyed by children to their parents, which teh school was worried about.

The reality of blackface, whether Border Morris style or Rochester sweeps, is very different, and I very much doubt if many people coming across it in its natural habitat, the street, ever see this as being anything to do with parodying black people.

Common sense in these matters is important - and it would have made every sense in a school performance to adjust things so as to avoid this kind of misunderstanding.

In fact it would have been basic good manners to make such an adjustment by switching to a different colour for the occasion. specially since there is no longstanding tradition involved which can be seen as a matter of principle.