As an instructor, I see many people that have issues clearing their mask. I NEVER certify anyone unless they can prove to me that they can easily clear their mask, multiple times, at different depths. If they can do it, but struggle through it, I may consider giving them their "scuba diver" rating which requires a dive master or instructor every time they go diving. If they can’t do it at all, I will NOT certify them. It’s one of those things that will easily freak someone out and potentially cause major problems.