This happens all the time. It's just not reported by principals wanting to preserve the false image that their schools are safe. And I personally know that this situation, and many almost as bad, with students offering physical violence or threats thereof, occur daily in NYC schools. Bloomberg started this trend by empowering students to write false statements which actually COULD lead to their teacher being fired. Now, adult professionals are not believed, and your students are your bosses. Obviously, they are not mature enough to handle this sort of power and it is abused daily. Told to put your cell phone away? Get a few of your buddies to write a completely fictitious statement that the teacher "touched you" or "made you feel bad". That's all it takes, especially if you're a veteran (read: "expensive and tenured") teacher, to be put up on totally false charges, have student "witnesses" be found totally credible and you and other adults speaking on your behalf not credible.
As far as physical assault, nowadays, teachers are on their own. Hush hush, sweep it under the carpet. Can't tell you how many times I was threatened, shoved, chest butted, had a physical object waved in my face, etc. Sometimes I'd come home covered in bruises. But I never backed down. To do so would have been inviting worse from the students.
THIS is what it's like to be in the classroom now.
Fire the lesser half of teachers and double the pay for the rest who can actually manage a classroom. And if any of them can’t, no tolerance, you’re gone.
(Firing all but one or two of the administrators would help as well...)