Usually, "cap and gown". But I have also graduated in a tuxedo.
Student or Learner
What do you call the costume that a graduate wears at his graduation ceremony?
Thank you very much!
academic gowns and hoods
In BrE, I've heard mortar board, not cap.
American high schools and colleges do use "cap and gown." The cap itself is called a mortar board, but the "costume" (uniform, outfit, whatever) is "cap and gown."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.