Oh ffs it's not sexism it's a perfectly normal developmental process. It's how kids develop their own sense of identity and self
I’m very much with @Beamur and @Stompythedinosaur on this. No, what was specifically described in the OP isn’t sexist, and children opting to play mainly in single sex groups isn’t an issue per se. Teachers shouldn’t be in the business of policing who should be playing with whom, unless it is directly impacting on the well-being of one or more of the children.
But I think what I and some other posters are recognising is the way that this gendered play can feed into early socialisation and the perpetuation of gender stereotypes, and this is an area where teachers and early years practitioners can wield some influence. This stuff - the idea that boys taking up more space is the natural order of things; the assumption that girls shouldn’t like dinosaurs or football, or that boys can’t do art or ballet - develops via parental and societal influence from a very early age, and schools and early years settings can often end up perpetuating stereotypes unless they’re very alert to how young children are being influenced by their environment and peer interactions.
Boys shouldn’t be returning from school and describing girls en masse as ‘bossy’ or ‘chatterboxes’ (‘bossy’, in particular, is a deeply gendered term). Girls shouldn’t be describing boys are a group as ‘naughty’ or ‘yucky’. No one group should be allowed to use space at the exclusion of others (at every single school I’ve worked at football-playing boys dominated the playground unless there was a designated no-ball or girls’ football zone).
And there are value judgements attached to these ideas. Never, throughout many years of KS1 parents’ consultations, did I come across a parent who was concerned that their daughter enjoyed football or construction play. However, I did have several who asked me to discourage their sons from using the art, role play or even Lego area because (and one father did actually use these terms), they were concerned their son were ‘too effeminate’ and not engaging with ‘proper outdoor rough and tumble’ with the other boys.
It’s a tricky one for schools and nurseries to recognise and tackle, because societal influences can be subtle and insidious, and gender stereotypes are still so deeply embedded in our culture (I’d actually argue that in many ways we’re going backwards in this regard). Just the other day a friend was telling me how her 3-year-old Encanto-loving daughter now refuses to sing along to ‘Surface Pressure’ because ‘Luisa is too strong for a girl and her muscles are too big’. Schools shouldn’t try to police friendships, but they should ensure that all children have equal access to a range of play opportunities, space is shared equally between the sexes and negative gendered language, however subtle, is discouraged as much as possible.