Well, if you take art history courses, you are taught that Renaissance artists used their own surroundings and notions in their art because they actually didn't know and you are told the same in art museums. We have no actual representations of what specific Jewish people, such as Jesus, looked like because Jews didn't depict humans in art at that time. It was forbidden. They would not have eaten in the manner shown in this artwork. If it's Passover they wouldn't have been eating bread, and eel (studies have revealed that this is what one of the food shown is) isn't kosher at any time. If people were serious about using this painting to teach about cultures, art, religions, history, etc., they would be pointing these things out. I don't think that's what they plan to do.They probably actually would know what things would have looked like in 1st century CE. You got to remember renaissance art is based around going back to the Greek and Roman traditions of art and away from the flat Middle Ages version of it. Most Renaissance paintings and art is based around humanistic expression and not based around factual interpretations of the subject matter. There are several depictions of Jesus in famous works of the time that have the Italian countryside or the artist’s local town. Places we are fairly certain Jesus never went. Also in nearly all depictions of Jesus at the time, they depict him as a white European person. Everyone at the time of Renaissance knew what a Jew or Middle Eastern person looked like, certainly within the merchant class.
My point is that most of these famous paintings from the Renaissance era, specifically the Italian Renaissance, depicting religious iconography are less about depicting accurate accounts and more about expression. If anything they are more focused on depicting their own communities and lives after centuries of suppression by the Church.
But the interpretation according the lesson plan seems slanted to a specific conclusion. Personally I don’t find issues breaking down a painting of the last supper if it is a renaissance lesson because it is an opportunity to have alot of inferences, but this does seem like a twisted Sunday school lesson that is disguised as public school curriculum.