He should have read the letters and made sure what he was sending out was appropriate. If he found some that were not - as there obviously were - he should have pulled them out an explained to the young letter writers what is wrong with them.
The kids could have raised their concerns about civilian populations without being offensive. Writing effectively used to be taught in school, right?
According to the dad the teacher DID read the letters & sent them off anyway. The teacher probably either agreed with the letters, or helped with the "writing" of them, ro both.