Proper range of motion for the shoulder is essential to perform movements like the overhead squat and muscle-up. Often athletes are limited either by tightness, weakness or a combination of both in the muscles surrounding the shoulder joint.
Here are two auxiliary exercises from Julien Pineau at Strongfit that you can add before and after class to help improve the positions and strength in your shoulders.
The key here is that you are strengthening the triceps, lats, and biceps under tension at end ranges. Start off with light weight and don't push the positions further than what you are capable.