The issue is not about whether a HS coach can dictate your taking lessons, its about staying healthy and at peak performance. As a position player you could spend time in a lesson and then go out and play a game without injury. However, this question is specifically about pitching lessons. Off-season, lessons are no problem. In season, pitchers need to follow specific throwing and conditioning routines for pre-game and post game days. If you are not a starter in a rotation, then you're jeopardizing your game performance and possible injury. I still say coordinating any outside pitching lessons with your HS coach is the way to go.