I agree with Steve and David. If possible, I always feed after a class rather than before. Waiting a couple extra hours is not going to hurt her. I did make an exception Halo's puppy class, it started at 11:00 AM on a Saturday, and by the time class ended and we drove home, she would have been getting breakfast between 1 and 2 PM, depending on whether or not we stopped for lunch on the way home. Since she's always been ridiculously food driven I went ahead and gave her a small meal first thing in the morning. Didn't make a bit of difference, she was just as willing to work.
For a less food motivated puppy I might have just waited and fed after. For evening classes I always fed after since the dogs aren't used to a super regular dinner schedule anyway.