In this case, although the letters are emails, they should be formal because a teacher is communicating with a student's parents about what is happening at school. Being emails, the sender does not have to write the full address of the person being written to. The first paragraph should say why you are writing the letter. The middle paragraphs should go into the details. The last paragraph should mention what the next steps would be and who needs to follow up. End off on a positive note if possible. The overall tone of the letters would depend on what you have decided the teacher is writing about in the first place. And, how the parent responds. Remember - there are points for being creative. Whatever angle you choose, make your letters effective by being clear and concise.