It's a very simple thing. You have
her parents call the school's principal and demand that the school take action to stop it.
If they do not, her parents can sue the school - since there is plenty of legal precedent (including Supreme Court rulings) that schools have some responsibility to prevent what you are describing on their grounds. Her parents should mention that they will file a complaint against the school and the principal with the federal Office of Civil Rights (Title IX).
That might actually result in ACTION - because schools fear lawsuits, especially ones that are very likely to go against them. You making stupid threats and mentioning firearms will not help your girlfriend, and will likely get you thrown out of school.
Also - MAKE SURE that this is actually happening, and not
(a) in your head.
(b) something your high school girlfriend is saying to get you riled up.