I am very sorry to say that 13 years after PADI published the article advocating teaching OW classes with students neutrally buoyant and in horizontal trim from the start of the class, there are still countless instructors teaching on the knees like this, and, even worse, they don't even know there is any controversy about it. I just saw a dive operation in South Florida advertise its instruction on a FaceBook site, and all the students were doing skills kneeling---and this was on their OW dives, not an early pool session. I made a comment about it, of course, and their response was "What's the issue?" They had never heard of doing it any other way and didn;t know whay it was a problem.
But let's talk about the mask clearing issue, because it is a prime example of why you should not teach that skill on the knees.
We tell students that they are supposed to tip their heads back when they clear the mask. Why is that? Because the mask needs to be perpendicular, with the bottom skirt of the mask needs to be the lowest point so the water will run out properly. But that is only true if the diver is in proper diving form! If the diver is kneeling, the mask is already perpendicular--it may even be past perpendicular. Tipping the head back is counterproductive. When the student is taught this skill in horizontal trim, then the head must be tipped back, or the water will accumulate in the bottom of the mask and stay there. Divers who learn the skill while kneeling will typically swing their entire bodies to a vertical position to clear the mask.