J-Kater has a good point.
When I did my advanced open water course a couple of years back in the Philippines, the first thing the instructor had us do once we were in the water was practice this skill. On the navigation dive a day later we finished the skills portion early, so he asked us to do a mask clearing again, plus a scheduled practice buddy breathing test as well.
My fiancée and I do these practice tests at the beginning of each trip, just as a precaution, just as many people do.
In Seattle for my rescue diver course, the instructor was interested to find out that I had done all my diving in warm tropical settings. He told me to do a mask clearing exercise in very shallow water in the Puget Sound near Alkai Beach. That freezing water was unlike anything
I’d ever experienced. It flushed over my face with a sensation similar to one of those freezing headaches you get when you eat ice cream too quickly, only much more intense. To my surprise, I stood up when this hit me. I quickly realized that he was wise to ask me to do this in shallow water before we hit deeper water later on for our rescue sessions. Even when skills are well practiced, the environment changes the dynamics of what you know and how you normally react.
As far as I am concerned, any further classes I take ought to come with a built in expectation for mask clearing among other things, not unlike the required check out dives many shops insist upon before taking even the most experienced divers out on deeper, more challenging dives.
Cheers!