Off-topic on a silly debate, but as one of the few resident 80s kids here ... I think the movie tries for a timeless feel, and has some obvious 80s inspiration. But the giant Spirit pop-up store selling slutty costumes is definitely a post mid-90s trope. More tellingly, the kids on the bus flashbacks (being vague on spoilers) are certainly more consistent with the late 70s/80s than the 50s. In other words, I think this is a stretch.