Yes, it is legal no employee has the right to unilaterally on his or her own decide when work is too dangerous and that he/she will take leave. Employees simply can't decide when to not go into work. Obviously, if you feel threatened, you can stay home from work--but then have to take the consequences for an unexcused absence e.g. discipline, suspension, possibly termination. However, you can also report this student to the police--report the threat and press charges for the assault punching you. That is your right your employer cannot deprive you of the protection of the criminal law or your right to report crimes and threatened crimes.