It really depends on the local officials and school...In my city, if the mayor signed off on you being released from school for an emergency run in the city, you had to be 18yo. We had 1 firefighter in high school 18 and the mayor stated that when the tones go off, if he is in town and not in an exam, he is an employee of the city and allowed to be released.
I don't recommend leaving school for a call. You're there to learn, especially high school. If you're in college, its your decision, but I would clear it with your professor first so they know you aren't just skipping out of class.