At my cc, the percentage of classes taught by full-time faculty in the first summer session is actually higher than it is during the regular academic year. I don't know if this is a local quirk, but it's noticeable. (And I know that pay isn't the issue, since we have a flat rate we pay for summer courses: a full-timer makes only about ten percent above the adjunct rate for a summer class. It's certainly nothing close to pro-rated.)
I've asked some of the faculty who stick around for the first session – roughly, late May and all of June – and they always give the same answer. We have very different students in the first summer session.
These students are typically traditional-age students who live in the area but who 'go away' to four-year colleges during the academic year. They're home for the summer, and picking up a few classes here to save some money and depressurize their schedules. They take a class or two in the morning, then go to work. The hallways are buzzing at 8:30, but dead shortly after noon. They don't pick up again until around 6:30.
The professors I've asked about it tell me that the students in this session are much lower-maintenance than the usual bunch. As one put it, “I say it once and they've got it.” He compared it to what he'd usually only find in an Honors section. They're certainly lower-maintenance on my end – after the first couple of days, it's unusual to see any in my office. Plagiarism complaints are few and far between this time of year.
My impression is that the 'visiting' students are generally more affluent than their year-round counterparts, and they bring with them many of the advantages of that affluence. (Those advantages are many: they can work fewer hours; they drive cars that are less likely to break down and cause them to miss class; they often have better preparation from high school; they know the unwritten rules of behavior in a classroom.) Judging by the faculty response, many full-timers are hungry for students who make teaching easier.
Oddly, the dynamic only seems to hold for the first session. The July-August session is almost entirely adjunct. The enrollments then are considerably lower, too. I suspect that this has to do with vacations.
It's striking to me, since it's the polar opposite of the stereotype of summer school. The stereotype is that summer school is full of losers, nobody wants to be there, and the whole enterprise is a bit of a joke. Here, summer school – at least for the first session – is a sort of annual foray into upscaling. For a month and a half, we get the upper-middle-class kids who usually shun cc's.
Is this a local quirk, or have you seen the same thing elsewhere?