I am currently working on a short piece on a school, and I can say from my experience that the obstacles are many, from preproduction thru post. One way to eliminate some of them early on, which is explained in detail by Michael Rabiger's book, is thorough preproduction. Especially when documenting an institution, first sell your idea to the head authority. The first thing I did was write a letter to the principal. Email is does not catch their attention quite like a letter, and as far as phoning your pitch, no one wants a pitch to from someone they've never had contact with before. Write a letter, BRIEFLY explain yourself as a film/videomaker, and simply request an audience with them.
Although the principal took more than three weeks to respond, she thought my letter was very professional and innocent enough to at least hear me out. From there I was able to convince the principal, and with her on my side convince the staff, and with the staff on my side convince the parents, and with the parents on my side, ultimately, convince the children to participate. Definitely, pick up Rabiger's book! Its been a great help to me, especially when it comes to tackling preproduction!