A prison could be a great setting for a Zombie film. But the more important part of the script is of course your characters. Showing who they are through the actions they take. How do they react to the tuations they find themselves in? Can the audience relate to the characters? These are a few things to keep in mind while you are writing the script. Watching films and studying them is good but reading film scripts is probably a better way to see what others have done. Hear a couple of tes that I have found that have scripts available for reading.
http://www.mplyscripts.com/http://www.imsdb.com/Both tes have a Horror section. I haven't read many of the scripts yet myself but both have Romero's Dawn Of The Dead as well as other Zombie films. So check them out. Have fun with it and I wish you luck.