If a professor is absent, can he force students to go to that cl anyway?

My professor was absent on Thursday, but he sent out an e-mail saying we had an ignment to do in cl and to submit to the school's server. He also said that the time stamp on the file would act as a "log out" time so he knows when everyone left. The ignment took about 2 hours to do (it's a 3.5 hour cl) so I bounced afterward. It seemed incredibly stupid to just sit there for the remainder of cl. Can he penalize me / dock my grade for that? I should also mention that we were completely unsupervised; he didn't get a sub or anything.