Do your callbacks ASAP and skip class to do it.
I didn't take any Friday classes, and I'm sure Henderson is OK with skipping Friday's if you e-mail him.
OCI is game time. If you want to work BigLaw out of school, go all out - bid on 25-30 firms in your city and go to every event your interviewing firms offer. Schedule you callbacks early, and don't do two callbacks in a single day unless you are sure of your ability to interview for 8 hours straight.
Just like with law school applications, the early callbacks have an easier time of things.
Heck, I know some people that have did callbacks before OCI started if they wanted to be in the city that they stayed at during the summer. This is a great idea, although difficult to pull off and not necessary.