I tell my students that in art the x-acto knife is a tool but out of the
class it is a weapon. It one is missing they can't leave until we find it.
I am fortunate that I have drafting tables in my art room and two students
sit to a table. I have the tables numbered and the knives numbered and I
give them out one at a time and take them up one at a time. They are not to
walk around the room with them and if they get up from their seats they have
to make sure their partner is watching their knife. If one is missing we
have had to call the principal before and it showed up right away.