To prevent such troubles, I take a two-pronged approach. If I am sending either good news or informative news home, I use a postcard:
This way when the kids pull it out of the mailbox they know that they will not get into trouble, and their parents actually get to read it. I ordered these personalized postcards from Paper Direct - very cute designs, and good sales for reasonable prices. I also like having them pre-printed with my name and school address, phone, email, etc - that way I don't have to write any of those things on there! Above is an invitation to Parent/Teacher Conferences, which I send out to every student's parents. I print the dates and times on a sticker to save extra work, but I like the personal effect of a handwritten note. I obviously start these WAY before PTCs.
However, let's say I'm sending bad news home - this I don't want displayed on a postcard. So I send it in a regular notecard with a regular envelope:
This way the kids don't know it's from the school, and the message gets through. I used Paper Direct for these also, and again my info is already printed on it. Sneaky!
In addition to invites and "bad" notes, I try to send at least two good postcards per class per week, to help build those positive perceptions. A lot of parents have commented on how much they appreciate "the postcards." Bonus - my parent attendance at conferences has gone from about 3-4 per night to around a dozen since I started this. In light of the total number of students I have it's a small percentage, but three times more than usual is a big improvement!