Factual error: The teacher in shop class is reading a newspaper with a headline about the Jim Jones suicide cult in Guyana, saying bodies were found, etc. However, the cult did not take the cyanide Kool-Aid until November 18, 1978, the concert in Detroit takes place October 13, over a month before the Jim Jones suicide/massacre.
Factual error: The Pizza Pig delivery driver arrives to the parochial school in a pink Lada Niva. Russian Ladas have never been imported into the States, and though it may be argued it have been purchased and brought into the US from Canada, since Canada did import them for awhile, the Lada is a late 80's model, and the movie takes place in 1978.
Factual error: When Beth is looking out the school window at Jam's Mom driving away with him leaving the school, at the top of the screen is an adjacent street and one of the cars passing by is a Chevrolet Venture minivan, which didn't come out until the mid 90s. (Note: this error has since been cropped away from DVD and Blu-ray versions and is only visible on the VHS release).