It's all about numbers and timing.
As long as the "hitting coach" sees one student at a time, he/she is, for the most part, not jeopardizing the players via OHSAA.
As for the Travel coaching High School, there is a time period when "no more than 4 players from the high school team may practice together"etc, etc, yadda yadda yadda... But otherwise, it's pretty open.
As TheBearMay aludes: read the fine print on the OHSAA website.