It's the same for private schools.
Listen to Los Angeles School Board member, Steven Zimmer, as he speaks out about class size (as a former teacher). It's a little less than three minutes long.
Heartfelt and inspiring. And yes, why is it that class size is touted for charters and private schools as a benefit and not public schools? Don't all kids deserve attention?
FYI, Bill Gates' kids' schools (former/current):
University Child Development School - UCDS is committed to small classes of 18 students and a student to teacher ratio of 8:1.
Seattle Girls Academy - Average class size: 22, Student/Teacher ratio 14:1
Lakeside - Average class size 16, Student/Teacher ratio 9:1