The Longitudinal Survey of Australian Children (LSAC) is following a bunch of kids born in 2004. (I tried to put in a link to it, but every link I found was broken. I must have tried 20 of them. It must have been taken down.)
Some researchers recently looked at the data about parental reports of kid behavior at ages 2 and 3 compared with income and child care status. They found a tiny association between being in child care and negative behaviors, mostly for kids of affluent and highly educated parents. That is, rich kids behave worse if they are in child care than if they are not. They also found fewer negative behaviors with smaller group size.
The researchers say it's a small difference and suggest it might be because of differences in families who chose child care, or more resources in rich homes, or differences in parenting across socioeconomic groups. As long as we're conjecturing about class differences in behavior, I'd add maybe rich parents have different expectations from "school" than poor parents, or maybe what counts as negative behavior.