My teaching experience is limited to private schools, so maybe it's different for public schools, but I can't wrap my mind around the idea of a teacher not knowing a student's address and home phone number. That is critical information for a teacher to know. S/he may need the number so she can call mom to explain that Junior is failing Spanish. Or she may learn from the address that Junior is failing Spanish because he's sleep deprived, thanks to that long bus ride from Outer Suburbia.
Of course it's all kinds of wrong for the teacher to use that info to woo clients for his/her side business, but to mail a thank-you note to a student? Not even in the same ballpark.
BTW, when I was teaching I never did have occasion to mail thank-you notes to students, because I thanked them in person. I did, however, get (snail mailed) thank-you notes from ex-students for the graduation gifts I gave them.