Drag queens are usually extreme representations of women, as someone said upthread - a pantomime dame approach. Personally I don't like them - but would never suggest banning them.
But are they appropriate to come in and read to children as a group? Men dressed up as grotesque parodies of women?
There must be thousands of different 'trades' - people who could be invited in to read to children - window cleaners, taxi drivers, cooks, nurses, police officers, musicians, SAHMs, senior citizens, gymnasts etc etc.
But none of these groups are 'edgy'. They may be completely appropriate and represent only good things in society, but they don't "turn on" this headteacher in the same way that drag queens do. So they won't get a look in.