Slightly different thing but when my son was at nursery, he spent the best part of six months when the first thing he did was head to the dressing up box and put on this cute little gypsy dress they had there over his normal clothes. We ended up buying him two dressing up boxes for Christmas, one aimed at boys, one at girls. Sometimes he'd be a policeman, sometimes a fairy.
Small kids have no idea about gender stereotypes. Let them wear what they want (within reason) and figure things out for themselves. They shouldn't be pressurised to wear anything they don't want to, neither by a parent, and definitely not by a teacher.