In the teacher's lounge, you can see the date "April 3, 1978" on the chalkboard. Later in the episode, Will asks Holden what the interviewee is listening to. Holden replies "Bittaker and Norris". Referencing the killers Lawrence Bittaker and Roy Norris. These men killed five teenagers in the time span of June 24, 1979, to October 31, 1979. Making it impossible for them to have tapes on the killers.
When the parents are speaking with Holden Ford about the principal's unusual method of punishment, the mom's manner of speaking doesn't jive with the 1970s. She speaks in a rapid-fire fashion with lots of vocal fry. This is a speech pattern very common with young women in the new millennium, but anachronistic in the 1970s.