Oh, due to the higher velocity? 1330 fps instead of 1200 for a "target load?"
Maybe. But this 28 gauge shell has 1/8 ounce less payload, which means 60 fewer pellets if both gauges had #8 loads.
The 28 ga field load uses more powder than the 20 gauge. It has to do with energy and area of the piston transferring the energy to the shot column. 20 is more efficient.
Have you or your students actually shot a 28 and a 20 side by side? Weight of gun and fit have more to do with comfort. A light gun is easier to carry, but kicks harder than a heavier gun. A gas gun will always be softer than a pump or O/U or even a recoil action auto. A short barrel is more uncomfortable than a longer barrel.