In our area, the coaches or the team usually make that decision. HS teams are usually not so stacked that they can afford to leave someone on the bench that can hit the ball. On summer teams, they often do separate out the pitchers.
Don't give up too early on either unless you are forced to. My son, early on was seen as just a pitcher. He hardly batted in all stars on teams that did very well. Now he is a soph in HS, splitting time between varsity and JV and is hitting .590. He still pitches, but he now is the top hitter (ave. and power) for the JV team and has hit .500 (6-12, 6 doubles) in his limited play with the varsity. I personally think that his hitting has surpassed his pitching, but not all see it that way. If he had to make the choice right now, he would play everday.