Of course, you need someone to keep an eye on the teacher.
A student, preferably the most trustworthy and well-thought of in the class, should serve as a failsafe. They will be issued a special box of colored pencils that conceals the detonator to the small, but lethal, amount of C4 hidden underneath the now-rogue teacher’s desk.
But even the trustworthy student could turn traitor and fall in line with the teacher, so a contingency plan must be in order for them too. Therefore the student immediately behind the failsafe’s desk will be in surreptitious contact with an AC-130 Gunship orbiting the school. If necessary, the secondary failsafe will get a hall pass to “go to the bathroom” and make contact with the gunship, which will target the offenders thanks to the GPS transponders embedded in the food they consumed in the cafeteria.
Of course, there is the possibility that the entire classroom will fall under the megalomaniacal sway of the teacher. In this case, the child located closest to the door will open up and strategically place the designated Hello Kitty backpack filled with Claymore mines.
And we must admit, there is the remote possibility that the entire class could be radicalized, leaving them with enough weapons for a respectable militia. In this case, the school principal will open the school’s safe and retrieve the “lunch box,” an actual lunch box that holds the activation codes for a miniaturized W78 nuclear weapon.
The safe also contains a set of keys to arm the device. In order to prevent further potential violence, the principal and his or her assistant will turn their keys simultaneously, eradicating any and all threats.