OH MY GOD! You cannot seriously be making this arguement.
No, you do not need to hang a flag to learn where a counry is or how its people live. It is called a teaching aid. Flags represent and identify countries.
No wonder American public schools are a laughing stock with sentiments like this.
BUT did it EVER occur to you that he could have LEGALLY posted large, full color PICTURES, equally a "teaching aid", of said flags on the room's bulletin board? When they actual part of the curriculum came up, THEN he could have temporarily LEGALLY displayed the flags, as well.
Instead, he chose first to hang the legally proscribed flags themselves; then willfully commit insubordination over the issue, rather than taking a very simple, LEGAL way out.
No, the reason American public schools are a laughing stock is that fundamentals of education like reading, writing and arithmetic take a back seat to indoctrination rituals of the liberal lunatics of the NEA and the AFT.