Agree with @ginti : tell the instructor your goals before the class, because breathing is not a particular focus of the class.Just be aware that fundie is a "shallow" course; skills are considerably harder at 6m than at 20m Therefore, your high SAC issue may not be an issue during the course. If you really want to work on it, be proactive with the instructor and ensure that he/she is aware of your limits.
If that helps, I have the same issue than you. For me, practicing drills a lot in neutral buoyancy until I didn’t need to think about your buoyancy too much helped me to reduce my sac while multitasking.