My DD1 is in Grade 5 this year. I reckon by about grade 4, so many kids catch the bus so you don't tend to see the parents at pick up and drop off. It was really noticeable this year as grade 5 is a big intake year at our school and I still have never laid eyes on some of the new parents! If DD wants to have a play with a child whose parents I don't know, I call and have a chat to a parent - usually mum - and always try to have the first play at our house!