My son's future school indicated they would prefer he not play in the summer (he's a PO). They would send him a throwing regimen to undertake beginning in July so that he's ready to begin pitching again in September for fall baseball. That feels right to me, since the fall is so important in terms of evaluating what a player's role will be in the spring. I imagine this is very pitcher-specific as there's nothing but upside for a position player to play the summer before school unless injury is a concern.