It varies some group to group in my kids' gym. There's a mix of early morning, mid-day, and later in the afternoon/early evening, but it tends to correlate higher levels with more frequent early starts.
Most working moms have help (grandparent, aunt/uncle, niece, etc). Some working moms adapt their hours and go in on a bigger carpool. Some hire in-home nannies or hire transportation.
I'm actually working on my teaching certificate and choosing to switch to that career path in part because of gym... I mean, the field interests me (though several do), and it works for their summers off in general, too. But the fact that it works with gym doesn't hurt!