The answer to the question “Cassock : Priest :: ? : Graduate” is “Gown”.
A cassock is a long, loose-fitting outer garment worn by priests and other clergy. A gown is a long, loose-fitting outer garment worn by graduates. Both a cassock and a gown are formal garments that are worn on special occasions.
A cap is a head covering that is worn by many different people, including graduates. A tie is a piece of clothing that is worn around the neck. A coat is a piece of clothing that is worn over the upper body.
While a cap, tie, and coat are all common garments, they are not as closely associated with graduation as a gown is. A gown is the traditional garment that is worn by graduates at commencement ceremonies. It is a symbol of the academic achievement that the graduate has attained.