It seems unrealistic that Danni would be running the soda fountain shop all by herself with that many customers and no one to help her. A lunch special is shown on a wall board. However, there is no one else shown doing any cooking, attending the counter, or helping her in any way.
Though set in 1965, virtually every school bus in the film was manufactured in the late-1970s and early-1980s. Most of them were built on truck cowls that didn't exist at the time, and contained features such as amber flashing lights next to the red ones above the windshield, which didn't exist in New York State until 1973.
Though set in 1965, a 1967 Pontiac is visible on the street outside the schoolyard.
When Dunn discovers Boo laying in his bed with all the flowers around her, the position of the baseball cap on the bedpost changes.