Second, this isn't a classroom, where you get an F for not showing up. In politics, you get an F for actively damaging things. Obama hasn't done that. He's simply ignored trade as an issue. But he hasn't done any harm, and under the circumstances that's quite possibly about as much as a trade enthusiast could have hoped for.
Drezner defends his judgment:
The U.S. political system is arranged to make it very difficult for anyone to change the status quo. Even if Barack Obama wanted to pull the United States out of the World Trade Organization, for example, he likely couldn't have gotten the necessary votes in Congress. The Obama administration has mildly resisted more hawkish member of Congress to "get tough" with China. That's about it in terms of preventing protectionism. When I said Obama had done almost nothing on trade, I wasn't kidding.