I just had one of those today. If it's in my own classroom, I keep calm and call security. Unfortunately, security never comes, but it gives the child time to calm themselves down. Today it happened while covering another class, this student was pretty confrontational. I turned on the charm, flattered him, and eventually got through. My thought was, I was in the far end of the building, by myself, the student was significantly larger than I am, and I had a lot more to lose than he did at this point in the school year. So definitely depends on the situation.