The Redeemer: Son of Satan! (1978)

Six former classmates receive invitations one day to a high school reunion. When they arrive at their alma mater, however, they find that not only are they the only ones to have receved letters, the invitations were actually sent by a deranged preacher intending to murder them all as punishment for their wicked ways. Will any of them escape from the remote schoolhouse alive, or will they all meet their final judgement?