The US officially entered the First World War in 1917. On the Western front, until the truce of November 11, 1918, this war was fought on French and Belgian territory. Therefore, it is improbable if not impossible that US soldiers (wearing British helmets) could have shot German civilians (meaning the parents of Helmut) in the street of a town.
Watched this on FreeVee streaming in 2024 and the audio sounds like it's in a tunnel.
When the students are saying the Pledge of Allegiance in class, they are holding their hands over their hearts. Until 1942 the prescribed body posture was to hold the right arm straight out, palm down, while reciting the Pledge. Congress authorized this change because this "American salute" was identical to the Nazi salute.
When we see the envelope Mr. Conlon is mailing (04:38), it shows the 2-letter state abbreviations for Washington and Massachusetts. These weren't adopted until 1963, when zip codes were introduced. More likely, the abbreviations would have been Wash. and Mass.
Betting on the point spread in a sporting event is a relatively modern invention, not in use at the time this film is set in.
Some of the school boys have belt loops on their trousers. Belt loops were not invented until 1922.
(At around 43 mins) Peter Coyote's character "Martin Conlon" is told he has received a letter. He goes into the small grocery goods store to collect it and the female employee calls his character "Peter" instead of "Martin."