The Sadist (1963), a low-budget film written and directed by James Landis, is about an escaped convict and his girlfriend who hold a group of teachers hostage in a junkyard. Despite the film's limited means, the basic situation is so well-rendered that it creates a constant tension throughout.
More information about the film can be found here:
http://en.wikipedia.org/wiki/The_Sadist_%28film%29