Yes, they had. The police had them on tape committing a conspiracy to attack the teacher. That's a world of difference better, legally, than someone calling the police and saying "so and so is threatening me." They also could have set up a sting to get them in the act before doing any damage but allowing enough of a confrontation to add the charges of assault and battery.
Yes, but in England things seem to be worse. People get in trouble for defending themselves from obvious crimes.