There is some truth in this. There's a psychological phenomenon (I'm too lazy to look for the correct name for it right now) that describes how people tend to internalize negative expectations. In mixed classrooms the pupils are often subjected to gendered stereotypes that cause for example girls to internalize that they aren't as good in mathematical subjects as boys and as a result they don't put as much effort to these subjects strengthening the stereotype. In girls only schools the girls have better results in mathematical subjects when there isn't as much pressure to conform to this stereotype. The boys on the other hand aren't expected to behave as "nice" as girls and thus their general school success suffers.
It's called stereotype threat. It's not clear it's a real thing (there are some failures to replicate, etc.) but that's about standard for social psychology. Either way, yeah, studies exist. "Studies exist" is a pretty low bar, but that's the claim she made and so that's the bar she has to clear.
I actually find it quite surprising she'd be willing to say segregation might have positive effects, I assumed that was SJ taboo. But then there are moves towards segregationist-ish things from the opposite end in some SJ circles, so who knows.
