The trivia item below may give away important plot points.
The death of "Maria" was a re-shoot. The director did not approve but agreed as it was a direct order from Roger Corman. Knowing this also helps explain continuity errors in the film. For example, as Maria is attacked the three other girls rush to aid, but presumably just stand there and watch her be molested and murdered while doing nothing. Originally after falling to the floor she was immediately killed ( as evident in the long shot of her murder. Maria's dress is still on. )