It doesn't matter if it's a pop quiz that involves measuring shapes, or the chance their fellow students have all turned into brain-hungry zombies. The last thing you want is to be chastised at the next parent-teacher meeting for not properly equipping your young'n.
So for just $20 this 30cm ruler will have them prepared for almost any perdicament. Although, like my father always said, "if it's a problem that can be solved by swinging a giant blade, it's not a problem." That man solved a lot of problems before the FBI finally caught up with him.