Swimming lessons at Tufts are one on one. One teacher for every kid. They don’t advertise this at all on their site.
The MIT lessons, which are more well-known, are 5:1, students to teachers. At this level, the lessons do need to more or less be one on one, so there, students spend a lot of time hang out while the teacher works with one of the students.
We only tried Tufts because it’s closer to us. Otherwise, we would have never know. I usually assume that information like this is readily available, but quite a bit of it is still hidden.