On the last day of school in the movie, the stoner kid named Slater was helping another student make a bong in woodclass, but later in the movie, the same kid he helped and another guy were smoking out of a red acrylic bong, instead of the wood one he made earlier that day. Acrylic is a plastic material that they started producing water pipes out of in the 1990's, not holding on to the 70's based era it's suppose to take place in.