An earlier poster mentioned that you can leave at noon if you want and no one notices...
Don't believe it. I'm don't know the particulars of that program, but residents and attendings notice when med students go missing. Yes, I put it on a student's eval when I was a resident. And I've noted it as a med student rotation coordinator. We actually had a student state on his second day that he wanted to be done by noon every day. When we told him this wasn't realistic, he didn't bother to show up the third day. Good riddance, I say.
I've read hundreds (thousands?) of recommendation letters -- the students that stay late when they don't need to, take extra call, are around and interested -- that goes in the letter. The ones that leave at noon, don't get good letters.