I had this long post written out and changed my mind.
No, I wouldn't have thrown a tantrum and made the principal change the classroom. No, I don't believe we have the whole story, just some assumptions. OP says her 'friends' told her they saw the grades and at least 50% were bad. Bad is relative, were 50% less than an A or 50% less than a D?
Do I feel badly for your son that he was embarrassed and hurt over his grade. Yes, and I definitely would have taken the opportunity to stand by my son with the teacher and explain to her that it was unacceptable. If need be I would request a conference with teacher and principal present. Teaching my child that you stand up for yourself in the correct way.
And after raising 10 children, boys are not tougher or different. Yeah, they might have different needs in certain ways, but to say boys get a pass because they are boys is ridiculous. Boys will be boys is the dumbest thing I ever heard of. They need to learn to control themselves and work hard just like their female counterparts. I also want to throw in I am not one to take what my children tell me about parents, teachers anything as 100% accurate. Notice I didn't say lying. Kids have the sin of omission. They see the big picture and miss the details of the situation until they start to mature.
OP, I don't agree with showing all the grades to everyone. But I heartily disagree with how it was handled. As for the whole beat the teacher up because 1/2 her class is failing according to the other parents, I am not buying it. Sounds like a lot of drama and assumptions. I don't worry about others kids grades anyway. The OP basically made this sound like a craft project anyway as opposed to any learning project. And all the posts regarding confidentiality etc. I never knew that. I know at college orientation we were given the speech, "Your kids are grown, grades are theirs the bill is yours'. I trusted my college age students enough to know that they would work hard, because they trusted I would yank their butt from college and not pay anymore if they didn't remain responsible. So, its interesting to note that grades are confidential information.
Kelly