An excellent skydiving school should be committed to both education and safety. They must have up to date facilities and reliable equipment. Check how long the school has been actively teaching skydiving. The number of years can give you an idea how good the school is. A bad school will not last more than a few years.
Check their track record and reviews online from past students. Most well established skydiving schools can provide some of the best and scenic skydiving spots on a daily basis and provide added services like hotel pickups. Make sure that their skydiving instructors are licensed.