Not to sound too cynical, but what, are you, crazy or something??
My respect for your volunteering to help. My first question is "what is the skill level of the JV coach?" Since he is your new boss, I would suggest you ask him every day what he wants you to do until you get to know each other. Is he good at baseball, has he coached before. Does he have a set routine, you hitting infield, he hitting infield, etc., etc.
Teaching the kids shouldn't be a problem for you, if you know baseball. If one way doesn't work, try another way. Teaching them the way the JV coach/senior coach wants may be a problem. Motivating them may be a problem. Getting them to work as a team may be a problem. The school administrators will probably be a problem. I can assure you, the parents will be a problem
My best advice is to make sure that the team has respect for their schooling, baseball, the coaches and each other. If you can teach them that, you should have a successful year. Good luck, it should be a rewarding endeavor.