Hi Beth,
I have taught physics laboratories and an astronomy course on solar system exploration. I really enjoyed teaching the astronomy course because it gave me the chance to share recent advances in space sciences with students and to talk about topics related to my own research. However, most of the time I concentrate on my research and teach through outreach programs like Solar Week.
I think that most scientists will agree that even though we have advanced degrees in our fields, we are still learning new things every day. The university where I work holds weekly seminars in which speakers from other universities and laboratories will come and talk to us about their research. These seminars aren't really like a class, since we don't have homework or tests. They are an opportunity to learn about new research topics and discuss our research with colleagues.
Depending on their job responsibilities, scientists and engineers may occasionally take classes or attend professional development workshops to keep their knowledge up-to-date and learn new skills. For example, I am currently taking courses in computer aided design (CAD) at the local community college. People who build spacecraft instruments use CAD software, so I felt it was an important skill for me to learn. My university also has workshops to help its employees write grant proposals or identify sources of research funds. Taking these types of classes is usually optional, and most of the time you do not have homework and grades like you do in high school or college. These classes are offered just to help people to do their jobs better.
Kris