Personally, as long as the teacher didn't give specific names I don't think it's a big deal.
My mother used to be a teacher and one of my cousins is a teacher, and they will openly talk about how bad or stupid their students act. There shouldn't be anything wrong with publicly saying bad things about your students in general.
And although I'm in my late 20s, I can clearly remember how me and my classmates would act in our classes. There was little respect for the teacher, and seeing some of the things written for homework or tests by students who simply didn't care about that subject, it could be scary for a teacher.
Plus, I'm sure most of you can remember talking equally bad about some of your teachers. Now as long as this was a personal blog, and not like a blog put on the schools website, I find nothing wrong with this.
Being a teacher in the US these days, you have little you can actually do about your students. To say that a teacher can't even talk negatively about their students in general in public is laughable.