It's not being meant in a mean way - it's a school-related thing, and is being applied to a group, with no reference to their ethnicities.
Backstory for my own emotional starting point here: for example, a couple of years ago an older man commented in a definitely patronising way that it was lovely to see my children "running around like little leprechauns". He wasn't trying to be rude - it was the casual sort of "you're very well spoken for a [insert ethnicity] person." kind of vibe, but the turn of phrase already had a negative twinge for me before it came up in school recently.
Just wondering how other Irish people in England would feel about their children's - let's say - ballet group - being called "Leprechauns" complete with stylised cartoon figure.
Am I being mad as a brush to be sensitive about it? Everyone else thinks it's delightful.