Reaching -- This will be my first summer as a law student, so I'm not exactly an expert, but I can tell you my plan.
I'm going to work at my current job, which is not really related to the law, through the summer. The larger firms in my city hire associates to work in the fall and spring, just like the ones they hire in the summer, so I'm going to quit my job in the fall and get one of those jobs. For the spring, I'll do another one of those jobs (if possible). If that's not possible, I'm going to try to find a job as a clerk or something in the legal field.
Other people in my class are doing different things. Many were clerks or paralegals at law firms before they came to LS, so they're going to just continue what they're doing. Most people have very flexible jobs with high turn overs (like servers at restaurants and retail jobs) that they can easily quit for the summer and go back to once school starts up again. A few people (we have a doctor and a physical therapist, for instance) are staying with their normal jobs at least until law school is over. I'm not sure what (if anything) that will do to their post-graduate career prospects.
You also asked about classes. At my school, there are no classes offered in the evenings during the summer. So, I think most of the PT students won't be taking summer classes. A few are going to take summer courses so they can graduate earlier or so they can transfer to the FT program in the fall, but that's certainly not the majority of folks.