I can understand parents' concerns but, in principle, he's just a "Mr" who wears a skirt. Unconventional - but again, in principle, how is that different from "Miss" who wears trousers?
What matters (IMO) is whether he's using the opportunity to promote his own agenda (if he has one). If he is solely focused on his job as a teacher then maybe it has to be accepted that 'traditions' change... as they did when women started wearing trousers to work.
Perhaps, also, it depends on what type of skirt he's wearing? If it's a 'sober' 'uniform' type of skirt, it would be perfectly acceptable (again, IMO). If, on the other hand, the skirt is of the more flamboyant "PRIDE" type of dress - then maybe that is not appropriate, considering that children are usually kept to a strict uniform regime themselves.
Hithere Here we go
Is there any reason why this shouldn't be a topic for debate?
If Mr was wearing a skirt in his private life I can see why it might be questionable to raise the point, but children's education is everyone's concern.