Difficult is etymologically related to Difficulty. Situation is a type of Difficulty. Exception is a type of Situation. 
Disorder is a different concept from Organization. Organization is related to Teacher. Teacher is related to Head. 
Difficult --EtymologicallyRelatedTo--> Difficulty <--IsA-- Situation <--IsA-- Exception 
Disorder <--DistinctFrom--> Organization <--RelatedTo--> Teacher <--RelatedTo--> Head 