There's a discussion going on about this at an Australian parenting site and I'm surprised at some of the comments tbh. I was wondering what MN thinks.
www.everybump.com.au/community/topic/7784-scrunch-butt-leggings-when-teaching-children/
I don't think that they're inappropriate at all and recoil at the idea of policing what women (especially young women) wear.
YABU: it's not appropriate for an educational setting.
YANBU: it's not a problem