How do you get a Gun Control Advocate to be against armed agents of the state?

Have said agent be a teacher.

But hardening proposals also exhibit a circular logic that runs deeply counter to the spirit of the Second Amendment. Again, that provision implies a duty to resist tyranny, in all the forms of military, surveillance, and governmental overreach that helped spark the revolution. Suggestions to create a police state in American schools, however, mirror other pro-authoritarian tendencies that run counter to this instinct. In the creation of the carceral state, in the expansion of drug laws, and in the extreme militarization of police in recent years, people have argued that placing more guns in the hands of authorities is the only way to keep people safe. But why would pro-Second Amendment enthusiasts be in favor of providing more firepower to the government?

Of course the teacher in this question isn't armed as a requirement for their job.  Nor is it an issued weapon.  The object is instead to give the option for someone to legally carry a personal weapon.

Though to get to that bit you had to read through a lot of drec.

It also ends on an interesting note.

One legal theory used to oppose the preferences of many defenders of the Second Amendment is based on the fact that the militarized American police state has advanced far beyond the ability of any possible well-regulated militia to stop it. But lost in that observation is the fact that Americans—many of them staunch gun-rights advocates—have pushed repeatedly to bolster the military and the creep of militarism into other civic arenas. 

A true point.

They’ve then trapped the country in an arms race between government and civilians, one in which civilians face severe losses from both state and private violence. And now students, protected in schools by the most basic tenets of the social contract, find themselves in the line of fire.

Except the social contract doesn't work if the police refuse to do their part...