In rule 2.13 (“Sweets”), at the end of “A Student may eat a Sweet they own, at any time, by removing it from their Tuck.”, add:

If that Student has no Cavity enchantment, he gains a Cavity enchantment with a Force of 1. Otherwise, he increments by one the Force of his Cavity enchantment.
If this would increase the Force of the enchantment to 6 or higher, it instead sets all the grades of that student to ‘F’ and that student’s condition is turned into a Zombie, dying of overwhelming magical cavities.

Create a new rule, named “Brushing teeth”, with the text:

Brushing your teeth is a weekly action and removes your Cavity enchantment if you had one.

Create a new subrule to rule 2.8 (“Spells”) named “Brosseae Àe Dentae”, with the text:

Difficulty Level: ALC(-3)
Effect: Removes the Target’s Cavity enchantment if it had one. If the Target is not the caster, the Target must join the spell, or there is no effect.
If the Caster’s grade in ALC is D, E, F or Ungraded, and if the Target is Alive, the caster must roll a DICE2 in the GNDT. On a 1, the target’s condition is set to Zombie.