Okay, wind your necks in, and read my initial response again!
Shaking with anger might be stretching it a little, but I would certainly be very cross!
If the letter was addressed to you, then it should have been passed to you, unopened. If your parents opened and read the letter, then put it in a new envelope, they knew they were not doing the right thing, and this should be addressed!
Have a gentle word with your parents, but also speak with the school, ensuring any future correspondence gets to you first, preferably by post (or email).
Don't fall out with your parents - they are obviously doing you huge favours by picking up your son from school (presumably it is a standing arrangement). Also, I expect they love and are concerned for your son's wellbeing, so perhaps imagined they were within their rights. Handle it carefully, and you will all come out the other side 
I understood from the OP that the fact your parents picked your son up from school was a standing arrangement.
If you trust your parents to pick up a child from school, then you trust them?
If not, then why do you allow them to do it? I presumed, as said in my post, that it was a standing arrangement. Now, reading these latest posts, from the "coven" (quite clearly it is not). As I previously said, however, if you do not want to hear people's views, why ask for them on a public forum???
I quite clearly stated that it was out of order for your parents to have opened your mail, and suggested solutions to try to resolve. It would seem you do not want your parents to have anything to do with your children any longer, so you do what you feel you have to do, according to your rules.
Good luck with it