Really guys? I'm not that stupid

So I gave my midterms today. about 10 of my 15 students copied everyone else's answers and I'm rather upset. They didn't even try to hide it. I haven't decided quite what to do about it yet. My plan for tomorrow was to give everyone back their graded midterms and go over them with my students, but as I don't have to have my midterm grades turned in for 2 weeks, I think tomorrow will be sitting in class and letting everyone write me a 200 word essay in English about why cheating is a bad idea, and what happens in my class if you get caught. An F is what happens, by the way. As this as is everyone's first offense this time, I may let it slide with just the essay. Either way, I'm still pretty ticked. I was starting to enjoy teaching and then they have to go pull a stunt like this...