Re: How would you handle this?
It is tough when a whole class gets in trouble for the actions of some. It is especially hard when you believe that your child is one of the kids who wasn't involved in whatevery the incident was.
However, children do not exist in isolation in a classroom. They are part of a community and sometimes it is the job of the entire community to police itself. A group "punishment" early in a school year often makes the rest of the year go much more smoothly. Your son is unlikely to suffer longterm damage.
I would assume that your son told her that there was a note from home and she told him fine. Perhaps his folder didn't get emptied and she never even saw what the note said. Personally, I would put the note in an envelope with the teacher's name on it to make certain that she gets it and realizes that it wasn't just the piece of paper leftover in his folder. I woul resend it like that tomorrow and then give her a day or two to get back with you. I think going to the administration or school board is way overboard unless there is something else serious going on.