The theme of this sorrowful story is teenage sexuality, particularly alternate lifestyle sexuality. In a way, it might be said that the central character Barbara "Sunny" Burhman serves to focalize the central attention on Zachary, which would in a sense make him the protagonist of the story. He is key to the story because Sunny's story is told in broad generalities until her life intersects with Zachery's life when he begins to intrude on her notice with his "Hi, Sunny!" In confirmation of this structural element, Sunny's life is once again covered in broad general strokes--her life after high school--until she encounters Tobias, then he speaks in detail, and he and Barbara interact in detail.
It is Zachary who chiefly presents the thematic element of alternate lifestyle sexuality, though we only know this when, in the protracted epilogue of the story (the epilogue that ironically gives the meaning to the title), Tobias confides to Barbara the secrets he has been hiding since high school. The story up until now has been Barbara's reaction to Zachary; in the epilogue, she learns what it is exactly that she has been reacting to: Zachary's confusion over his life, a confusion that extends outward from his confusion over his alternate sexual preferences.
If one were to try to make a statement of Oates thesis that underlies "Life After High School," the best summary statement of her thesis would be found in Zachary's last letter:
To whom It May (Or May Not) Concern:
I, Zachary A. Graff, being of sound mind & body, do hereby declare that I have taken my own life of my own free will & I hereby declare all others guiltless as they are ignorant of the death of the aforementioned & the life.
ZACHARY A. GRAFF
The line pertinent to a thesis statement to embody Oates purpose in writing is "they are ignorant of ... the life [of the aforementioned]." From this, one might deduce that a representative thesis statement might be: "While outward appearances give a shadow of the identity of an individual, each inner person is veiled beneath an opaque shield that darkens the truth of love and desire that lies in secret within."
Sunny stared up at him, for an instant mute, paralyzed, seeing not Zachary Graff's eyes but the lenses of his glases which appeared, in the semidark, opaque.