Nations aren’t considered gods, and flags aren’t idols of those non-gods. You don’t sacrifice goats to the flag, or burn incense for it.

Judaism has historically looked at Christian beliefs and practices with way, way more suspicion of polytheism and idolatry than it’s ever looked at national flags.

The teacher here is unhinged, but you clearly don’t really know very much about the ten commandments, especially from a Jewish context.