Dear Holly,
Interestingly, I just got the same question for the same reason (staff) from a camp that does trail riding. Certainly a sleeved shirt gives you a little more protection from road rash, but I do not think it could ever possibly be a true safety issue, since the shirt would never protect you from serious injury. In my opinion, you need to look at your own program and come to your own conclusions. I would say that the bigger issue, as you have touched on, is whether or not you want your staff to be good role models and live up to the Pony Club standard. It probably depends on how PC-ish your program is. Will the staff be expected to wear the shirts everyday? If they only have to wear them on special days (opening day and closing day), then why not make them wear a more appropriate shirt? If it is everyday, well, you have to weigh out all the factors: safety, role model, staff satisfaction, professional appearance, and make your own decision based on what is best for your program. My staff is expected to wear a short or long sleeved collared shirt on the opening day of any of our programs. After that, it is okay to wear a tank top as long as they look neat and clean and the shirt is tucked-in.

Good luck and keep up your high standards!