As a former classroom teacher, I want to add my two cents. (I taught upper elementary grades over twenty years ago, in mostly self-contained classrooms.)
Each year I would mail a welcome letter two or three days before school started to each student at his/her home address. I was not required to do this, but my principal had mentioned that he thought it was a nice idea. I cannot remember for sure, but I think we could send the welcome letter out through the school's postage meter.
I also mailed a personal thank you note (always using my own postage stamp) to each student who gave me a Christmas gift or end-of-the-year gift. I thought it was the right thing to do because I was modeling politeness and modeling correct letter writing. Besides, I thought most kids didn't receive much mail and would appreciate a personal thank you note.
I was so busy during the school days that I would write those thank you notes after school or at home in the evening. I still think it was the right thing to have done, but if I had known that an individual child's family did not want their child to receive a thank you note through the mail, I certainly would have respected their instructions.
[I did have mixed feelings about receiving gifts from students. I was being paid to teach; no gifts were needed, and indeed, I think I would have preferred to not receive gifts, because I wouldn't have wanted anyone to think that there was any preferential treatment (because there wasn't).
When I got married and had kids of my own I did not give gifts to my children's teachers. What I did do is to write thank you letters to their teachers, mentioning specific things my sons had learned in their classes that I was thankful for.]
So, OP, I think you were fine.
