Well I think it is ironic that you want to define irony to teenagers when they are the kings and queens of sarcastic irony.
Irony, sarcasm, satire indicate mockery of something or someone. The essential feature of irony is the indirect presentation of a contradiction between an action or expression and the context in which it occurs. In the figure of speech, emphasis is placed on the opposition between the literal and intended meaning of a statement; one thing is said and its opposite implied, as in the comment, “Beautiful weather, isn't it?” made when it is raining or nasty. Ironic literature exploits, in addition to the rhetorical figure, such devices as character development, situation, and plot to stress the paradoxical nature of reality or the contrast between an ideal and actual condition, set of circumstances, etc., frequently in such a way as to stress the absurdity present in the contradiction between substance and form. Irony differs from sarcasm in greater subtlety and wit. In sarcasm ridicule or mockery is used harshly, often crudely and contemptuously, for destructive purposes. It may be used in an indirect manner, and have the form of irony, as in “What a fine musician you turned out to be!” or it may be used in the form of a direct statement, “You couldn't play one piece correctly if you had two assistants.” The distinctive quality of sarcasm is present in the spoken word and manifested chiefly by vocal inflection, whereas satire and irony, arising originally as literary and rhetorical forms, are exhibited in the organization or structuring of either language or literary material. Satire usually implies the use of irony or sarcasm for censorious or critical purposes and is often directed at public figures or institutions, conventional behavior, political situations, etc.
Wow, like really, they are going to understand this.